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• How to set up raid monitoring on a Adaptec AAC-RAID on a Dell?

  - by pjz

  I've got Debian on a Dell with an Adaptec RAID: AAC0: kernel 4.1-0[7417] AAC0: monitor 4.1-0[7417] AAC0: bios 4.1-0[7417] AAC0: serial 4edf09 scsi2 : aacraid Vendor: CERC Model: DATA 1 Rev: V1.0 Type: Direct-Access ANSI SCSI revision: 02 Vendor: CERC Model: Data 2 Rev: V1.0 Type: Direct-Access ANSI SCSI revision: 02 I've got it set up using the fine afacli tool (ick). Is there a way to get it to email me when there's an error? Do I need to hand-roll a script for this? what's the right version of the afacli tool to use? v4.1 gets SIGABRT and exits after I ask it to 'container list' or 'enclosure list' v2.8's enclosure commands don't work either Do I need to upgrade firmware? to what? from where? how?

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• After enabling raid0, blue screen of death

  - by thegreyspot

  Hi! Im trying to get a raid0 setup going. I have a 750 gb hdd with the OS on it and two 80gb HDD. For right now, I just want to raid0 on the two 80Gb hdd. When I enable raid in my Bios, Win7 has a BSOD during boot. As far as I understand I need to install raid drivers. However how can I do this? The win7 DVD doesnt seem to have a driver install option. Any suggestions? Thanks EDIT: I saw that there was as seperate setting for the 4th and 5th sata ports. So I pluged in my OS drive into there and left it on IDE. Still didnt work though, am I miss understanding that idealogy?

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• How long to erase flash memory (RAID controller)?

  - by Rob Nicholson

  I made a bit of a boo boo last night in upgrading the BIOS in a Silicon Image Sil3132 eSATA adapter. It's a RAID controller in a server. I accidently flashed it with the wrong BIOS ;-) Not the end of the world as this card only cost £15 but I'm trying to flash it with the correct BIOS as it won't obviously work anymore. Silicon Image supply a DOS program for flashing and I thought I'd use the Erase function to get rid of the old BIOS. Any idea how long this should take? It's been sat at the "Erasing memory" prompt for about 15 minutes. Thanks, Rob.

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• Impossible to stop raid device

  - by Traroth

  I'm trying to stop a RAID disk in order to replace it by a new one, as this one is not working properly. I'm typing mdadm --stop /dev/md1, and I'm getting an error message: mdadm: fail to stop array /dev/md1: Device or resource busy I'm getting this message even if I reboot the server, and I can't see a process that could cause this. The server is working under a Debian with a 2.6.18-4-amd64 kernel. Could you help? Edit: Mote details about what my colleague tried out. After unmounting sda1, the command mdadm --remove /dev/md0 /dev/sda1 worked. But now, we still have an error message after mdadm --remove /dev/md1 /dev/sda5: mdadm: hot remove failed for /dev/sda5: Device or resource busy I still don't understand completly how the different partitions are mounted, so I suppose there is something I don't understand in the currenct situation...

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• win2008 R2 server core software RAID

  - by shimonyk

  I am trying to set up software raid on a win2008 R2 server core. I have the disks configured as dynamic. In the server manager gui, i can see the disks, but when i right click, the option to set up "new mirrored volume" is not listed. I tried it with the command line using diskpart, and it gives the error "Virtual Disk Service Error: The size of the extent is less than the minimum." The drive are a new pair of 1Tb disks. Is this not supported in server core, or am i missing something else? Thank you

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• Setting up Raid 1 Array for Home Server

  - by user1048116

  I'm not sure if this is even possible, but it's worth asking on here! Essentially I have a old machine at home (well, not old hardware wise, but I recently built a new gaming rig), which I decided to install a copy of W2008 R2 on and use as a file/backup server and media center'ish machine. As of now, it has a single drive partitioned into C and D, with D being the Data partition. I have happened to find an old 1TB SATA drive lying around at home, and was wondering if it's possible to setup a Raid 1 array in my rig within Windows without needing to lose everything on my first drive (or maybe even just mirror a specific partition, say the Data partition, as this is just what stores my photos etc). Maybe this isn't possible, but you never know :) Regards, T.C

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• Alerting when a RAID Array disk fails locally on VMWare ESX or ESXi System

  - by Tim K

  With ESX and ESXi, we recently had two systems where that the boot partition became degraded due to a failed disk. The only alert we managed to capture was the visual alert on the Dell servers. We failed to received any electronic alerts regarding the failed or degraded array. Does anyone have any experience with monitoring for these types of failures? In both cases, the servers were running in a RAID 5 SCSI configuration (5 disks on one system, 3 disks on another) which if we were running a Windows Server OS, we would have had an alert created in the Eventviewer. Where would I begin to look for this solution. Can it be configured in VCenter or vFoglight?

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• Expanding RAID 1 array size (Adaptec 1420SA controller)

  - by cincura.net

  I have an Adaptec 1420SA controller in server and RAID 1 array created. The old disks slowly started to report S.M.A.R.T errors so I replaced first one, rebuild array and then other one and rebuild the array again. But the new drives are bigger, so I'd like to expand the array to use full disk capacity. Is it somehow possible? In the Adaptec configuration utility, after POST, I didn't found anything that could do it. I still have the old drives, if it helps. The array is used for system (Windows 2008 R2 SP1) booting. Thanks.

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• Transfer Raid Drives to External Enclosure

  - by dubbeat

  I have 2 raid disks (a grand total of 360GB) in my laptop. I'm fast running out of space and want to install new drives. I've a pretty good idea how to do this. My question is what can I do with the drives that I remove? I've got lots of media files on these drives that I'd like to keep and maybe transfer back onto my laptop once I have the new drives installed. Bearing in mind that I know next to nothing about hardware how do you suggegst I go about reusing the removed drives somehow? Thanks,

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• FreeNAS/ZFS/Raid-Z slightly different disks

  - by muskratt

  I'm considering using FreeNAS and "recycling" some of my older 1TB disks. Two are the exact same model Western Digital while another is Seagate and the fourth is Samsung. Typically, since all disks are not equal, I'll create my arrays on a Windows-based server 1GB undersized to prevent a replacement disk not being large enough. Dell is notorious for sending replacement SATA disks of different brand---knock on wood, no problems yet. Since not all drives are created equally and they can vary a few MBs, is there a way to make the the FreeNas/ZFS/Raid-Z function in the same way I do for my Windows-based servers above? Thanks

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• Software Raid 10 corrupted superblock after dual disk failure, how do I recover it?

  - by Shoshomiga

  I have a software raid 10 with 6 x 2tb hard drives (raid 1 for /boot), ubuntu 10.04 is the os. I had a raid controller failure that put 2 drives out of sync, crashed the system and initially the os didnt boot up and went into initramfs instead, saying that drives were busy but I eventually managed to bring the raid up by stopping and assembling the drives. The os booted up and said that there were filesystem errors, I chose to ignore because it would remount the fs in read-only mode if there was a problem. Everything seemed to be working fine and the 2 drives started to rebuild, I was sure that it was a sata controller failure because I had dma errors in my log files. The os crashed soon after that with ext errors. Now its not bringing up the raid, it says that there is no superblock on /dev/sda2, even if I assemble manually with all the device names. I also did a memtest and changed the motherboard in addition to everything else. EDIT: This is my partition layout Disk /dev/sdb: 2000.4 GB, 2000398934016 bytes 255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disk identifier: 0x0009c34a Device Boot Start End Blocks Id System /dev/sdb1 * 2048 511999 254976 83 Linux /dev/sdb2 512000 3904980991 1952234496 83 Linux /dev/sdb3 3904980992 3907028991 1024000 82 Linux swap / Solaris All 6 disks have the same layout, partition #1 is for raid 1 /boot, partition #2 is for raid 10 far plan, partition #3 is swap, but sda did not have swap enabled EDIT2: This is the output of mdadm --detail /dev/md1 Layout : near=1, far=2 Chunk Size : 64k UUID : a0feff55:2018f8ff:e368bf24:bd0fce41 Events : 0.3112126 Number Major Minor RaidDevice State 0 8 34 0 spare rebuilding /dev/sdc2 1 0 0 1 removed 2 8 18 2 active sync /dev/sdb2 3 8 50 3 active sync /dev/sdd2 4 0 0 4 removed 5 8 82 5 active sync /dev/sdf2 6 8 66 - spare /dev/sde2 EDIT3: I ran ddrescue and it has copied everything from sda except a single 4096 byte sector that I suspect is the raid superblock EDIT4: Here is some more info too long to fit here lshw: http://pastebin.com/2eKrh7nF mdadm --detail /dev/sd[abcdef]1 (raid1): http://pastebin.com/cgMQWerS mdadm --detail /dev/sd[abcdef]2 (raid10): http://pastebin.com/V5dtcGPF dumpe2fs of /dev/sda2 (from the ddrescue cloned drive): http://pastebin.com/sp0GYcJG I tried to recreate md1 based on this info with the command mdadm --create /dev/md1 -v --assume-clean --level=10 --raid-devices=6 --chunk=64K --layout=f2 /dev/sda2 missing /dev/sdc2 /dev/sdd2 missing /dev/sdf2 But I can't mount it, I also tried to recreate it based on my initial mdadm --detail /dev/md1 but it still doesn't mount It also warns me that /dev/sda2 is an ext2fs file system but I guess its because of ddrescue

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• Is it normal for a SAS drive to have a few bad blocks, or should I replace my drive ASAP?

  - by Nate

  I have a drive—part of a RAID 1 mirror—that has two bad blocks. Adaptec Storage Manger e-mailed me when it detected the blocks. It shows 4 medium errors for that drive, but state is still “optimal”. This is my first time using Adaptec RAID controllers. I don’t know if an occasional bad block is normal, or if I should immediately replace that drive. Update: The drive failed later the same day! The disk subsystem is: Adaptec 6405 with ZMM (2) Seagate near-line SAS drives (ST31000424SS) The other drive hasn’t reported any bad blocks yet. I am running a consistency check.

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• RAID 1 Mirror Error with Two Western Digital 500GB Drives

  - by bm678

  I have Windows 7 Ultimate 64 bit installed on a Western Digital 500GB drive (WD5000BEVT-22ZAT0) that was partitioned automatically by Windows as 100MB System Reserved and 465.66GB drive C. There is also an unallocated second Western Digital 500GB drive (WD5000BPVT-22HXZT1) that I want to use for RAID 1 to mirror the first drive but I get an error message stating “ALL DISKS HOLDING EXTENTS FOR A GIVEN VOLUME MUST HAVE THE SAME SECTOR SIZE, AND THE SECTOR SIZE MUST BE VALID.” I uninstalled Windows patch KB-982018 but I still get the same error message. Could you please let me know how to resolve this? Thanks.

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• Raid system fails to boot after moving from 2.6 kernel to 3.5

  - by user846226

  I'm running Gentoo Linux with a custom compiled kernel and I've just migrated from a 2.6 kernel to a 3.5. As my HD's are on RAID 0 mode I use a custom initrd file in order to be able to boot. While kernel 2.6 is able to boot without problems the new 3.5 compiled kernel fails to boot complaining about no block devices found. After taking a look at initrd.cpio contained scripts I can see the failure message is given by mdadm tool. Does anyone has a clue about that? Edit: this morning i noticed there was some kind of issue on my old initrd which works fine for 2.6 kernels, so created a new initrd which works fine and let me to boot into GNU/Gentoo Linux with same 3.5 bzImage. It could be an issue of mdadm, gonna check it. https://bugs.gentoo.org/show_bug.cgi?id=416081

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• Access RAID configuration in x3550M2

  - by Mike B

  I'm trying to configure RAID in a IBM x3550 M2 server. I can't find any messages on boot about any hotkey to access the configuration utility. I wasn't able either in BIOS. The card is a on board LSI 1068e. I can't find the original CD's shipped with the server, but I downloaded Server Guide 9.21 from IBM web. Booted with that, and only getting stuck at "Windows loading"p. Tried with a 8.? version from a x3550 (M1), but it was less useful. Although I do not consider myself an expert, I'm starting to feel like a newbie luser. Any help?

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• Installing raid controller forces reinstall of Windows Server 2008

  - by Tyler

  So, I've tried two different RAID controllers that have external SATA connections on my Server 2008 machine. I can install the hardware, boot into Windows, install the drivers and reboot again. No problems. However, as soon as I try to use eSATA-connected drives and reboot something happens to the Windows install and I can no longer boot into Windows. I tried repairing from the command line, and the end result is that repair console tells me I have 0 Windows installations (?). I end up having no choice but to reinstall Windows to get back on track. I must be doing something fundamentally wrong here, but I don't know what :(

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• Windows XP 32-bit + RocketRaid 622 + 4 x 3TB = not quite a RAID setup

  - by gmoney

  I'm looking to make a 6TB RAID 10 array from my new pile of drives under Windows XP 32-bit, however they are only for auxiliary storage. After adding all the drives to an array, and initializing them XP sees only a fraction of the storage, 2TB. I'm assuming this has to do with MBR vs GPT. Is making a series of 2TB volumes and then spanning my only solution? Most questions online have to do with booting from this setup, but I'm just using them as extra storage. Hardware: 4 x 3TB Hitachi Deskstars + RocketRaid 622 + Sans Digital TR8M TowerRAID. The array is connected via eSATA.

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• What are the recognized ways to increase the size of the RAID array online/offline?

  - by user149509

  Is it possible, in theory, increase the size of the RAID-array of any level just by adding new drive(s)? Variant like "backup whole data - delete old array - add/replace disks - create new array - restore data" is obvious so what are the other options? Does it depend on the RAID-level only or on the implementation of RAID-controller only, or on both? Adding new disks to a striped array necessarily leads to a rebuilding of the array with the redistribution of the strips to the new drives? What steps should be done to increase size of RAID-array in online/offline scenarios? Especially interesting RAID-5 and RAID-10. I would like to see the big picture.

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• NVidia raid 5 array spooling sounds and delay

  - by Chase B. Gale

  I've had a raid 5 array setup with 3 2TB WD Green drives for about 3 years now. Starting last week, when I would access the array for the first time, I hear a loud drive spooling sound and experience a ~5 second delay before being able to access\save files. This behavior happens when I don't use the array for some time (about an hour) and after occurring it doesn't happen again if I continue to access the array. I've run SMART scans on all drives and they come back as being a-ok. What's causing this? Is my array getting close to death?

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• Windows shows incorrect free space on Raid 10 volume

  - by Adenverd

  I have 4 1TB hard drives in RAID 1+0 configuration. Theoretically, I should have ~2TB of available space. Windows says the drive has a total size of 1.81 TB, which I'm fine with. As far as files on the volume go, I used WinDirStat to determine that I have 552.8GB of files on the volume. This means that I should have somewhere around 1.3TB minimum of free space. Yet Windows shows the drive as only having 492GB of free space. Are there hidden files somewhere that I can't see (I have show hidden files/folders turned on)? Does Windows not recognize that old files have been deleted? Is there any way to correct this problem?

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• Setup access to SAS RAID drives with NTFS partitions on CentOS Machine

  - by Quanano

  We have a Dell Poweredge 2900 system with Adaptec 39320A SCSI CONTROLLER CARD and 4 SAS hard drives attached, with NTFS partitions on them. We installed CentOS on the other raid array with a different controller and it is working fine. We are now trying to access the drives shown above and they are not being shown in /dev as sdb, etc. sda is the drive that we installed centos on and it has sda1, sda2, sda3, etc. The CDROM has been picked up as well. If I scan for scsi devices then the perc and adaptec controllers are both found. sg0 is the CDROM and sg2 is the centos installed, however I think sg1 is the other drive but I cannot see anyway to mount the partitions, as only the drive is listed in /dev. Thanks. EXTRA INFO fdisk -l Disk /dev/sda: 72.7 GB, 72746008576 bytes 255 heads, 63 sectors/track, 8844 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x11e3119f Device Boot Start End Blocks Id System /dev/sda1 * 1 64 512000 83 Linux Partition 1 does not end on cylinder boundary. /dev/sda2 64 8845 70528000 8e Linux LVM Disk /dev/mapper/vg_lal2server-lv_root: 34.4 GB, 34431041536 bytes 255 heads, 63 sectors/track, 4186 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Disk /dev/mapper/vg_lal2server-lv_root doesn't contain a valid partition table Disk /dev/mapper/vg_lal2server-lv_swap: 21.1 GB, 21139292160 bytes 255 heads, 63 sectors/track, 2570 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Disk /dev/mapper/vg_lal2server-lv_swap doesn't contain a valid partition table Disk /dev/mapper/vg_lal2server-lv_home: 16.6 GB, 16647192576 bytes 255 heads, 63 sectors/track, 2023 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Disk /dev/mapper/vg_lal2server-lv_home doesn't contain a valid partition table These are all from the install hdd not the additional hard drives modprobe a320raid FATAL: Module a320raid not found. lsscsi -v [0:0:0:0] cd/dvd TSSTcorp CDRWDVD TS-H492C DE02 /dev/sr0 dir: /sys/bus/scsi/devices/0:0:0:0 [/sys/devices/pci0000:00/0000:00:1f.1/host0/target0:0:0/0:0:0:0] [4:0:10:0] enclosu DP BACKPLANE 1.05 - dir: /sys/bus/scsi/devices/4:0:10:0 [/sys/devices/pci0000:00/0000:00:05.0/0000:01:00.0/0000:02:0e.0/host4/target4:0:10/4:0:10:0] [4:2:0:0] disk DELL PERC 5/i 1.03 /dev/sda dir: /sys/bus/scsi/devices/4:2:0:0 [/sys/devices/pci0000:00/0000:00:05.0/0000:01:00.0/0000:02:0e.0/host4/target4:2:0/4:2:0:0] . lsmod Module Size Used by fuse 66285 0 des_generic 16604 0 ecb 2209 0 md4 3461 0 nls_utf8 1455 0 cifs 278370 0 autofs4 26888 4 ipt_REJECT 2383 0 ip6t_REJECT 4628 2 nf_conntrack_ipv6 8748 2 nf_defrag_ipv6 12182 1 nf_conntrack_ipv6 xt_state 1492 2 nf_conntrack 79453 2 nf_conntrack_ipv6,xt_state ip6table_filter 2889 1 ip6_tables 19458 1 ip6table_filter ipv6 322029 31 ip6t_REJECT,nf_conntrack_ipv6,nf_defrag_ipv6 bnx2 79618 0 ses 6859 0 enclosure 8395 1 ses dcdbas 9219 0 serio_raw 4818 0 sg 30124 0 iTCO_wdt 13662 0 iTCO_vendor_support 3088 1 iTCO_wdt i5000_edac 8867 0 edac_core 46773 3 i5000_edac i5k_amb 5105 0 shpchp 33482 0 ext4 364410 3 mbcache 8144 1 ext4 jbd2 88738 1 ext4 sd_mod 39488 3 crc_t10dif 1541 1 sd_mod sr_mod 16228 0 cdrom 39771 1 sr_mod megaraid_sas 77090 2 aic79xx 129492 0 scsi_transport_spi 26151 1 aic79xx pata_acpi 3701 0 ata_generic 3837 0 ata_piix 22846 0 radeon 1023359 1 ttm 70328 1 radeon drm_kms_helper 33236 1 radeon drm 230675 3 radeon,ttm,drm_kms_helper i2c_algo_bit 5762 1 radeon i2c_core 31276 4 radeon,drm_kms_helper,drm,i2c_algo_bit dm_mirror 14101 0 dm_region_hash 12170 1 dm_mirror dm_log 10122 2 dm_mirror,dm_region_hash dm_mod 81500 11 dm_mirror,dm_log

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• Problems migrating software RAID 5 to new server (linux)

  - by leleu

  I have a CentOS setup with sw RAID5 that holds my data. Well, the server died, so I bought another box to migrate my drives to. Only thing is, I cannot get the RAID array rebuilt (not even sure it needs rebuilding, might just need the /dev/md0 mapping created... but I don't even know how to determine what I need!) Some details: RAID5 software (used mdadm) 4x 250GB drives (2 are SATA, 2 are EIDE -- would this matter? It worked fine in the other box...) latest CentOS distro built using mdadm I've got a decent amount of experience with standard linux stuff, but the hardware level stuff runs me in circles. I've spent some time googling and elsewhere here on SF, so please be kind for my newbie questions :). My question is this: how can I diagnose the problem? For all I know, I'm using the wrong device blocks when I try to rebuild the array, but I can't find the command to display only the devices that have some physical attachment. Is there some simple way for me to run mdadm, having it scan over all my physical drives, and say "hey, drives 2,5,6,7 are a software array, want me to mount it?" I basically just took the drives from my old box and put it into my new one. They show up in the BIOS. What steps do I need to take in order to get the array up, running, and mounted? Thanks in advance!

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• Resize a RAID 1 volume on OSX Snow Leopard - how? (Note: software raid)

  - by Emmel

  I've scoured the Internet in search of an answer to this question, and as usual with OSX-related topics, I often don't find any deep-dive technical explanations sufficient enough to feel confident doing dangerous things. Here is my question: I have a Mac Pro, running OSX 10.6.2. I have, as my main root/boot disk, a RAID 1 volume called "Mirror1". Mirror1 is comprised of two 1 TB disks. Mirror1, however, is fixed at 640 GB. That's because, I originally took a 640GB disk, bought a terabyte disk, mirrored it (using diskutil appleraid enable...), when it synced I removed the 640GB and replaced it with a second 1 TB disk, and synced again. Voila! A single 640 GB replaced by two 1 TB disks in a mirror.. Actually, no. There's still something missing from the equation: Mirror1 needs to be expanded from 640GB to 1 TB to match the partition sizes on each of those disks. How do I do this? Perhaps the diskutil output will help: -> diskutil list /dev/disk0 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *1.0 TB disk0 1: EFI 209.7 MB disk0s1 2: Apple_RAID 999.9 GB disk0s2 3: Apple_Boot Boot OSX 134.2 MB disk0s3 /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *1.0 TB disk1 1: EFI 209.7 MB disk1s1 2: Apple_RAID 999.9 GB disk1s2 3: Apple_Boot Boot OSX 134.2 MB disk1s3 /dev/disk2 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *640.1 GB disk2 1: EFI 209.7 MB disk2s1 2: Apple_HFS Mac Disk 2 536.7 GB disk2s2 3: Microsoft Basic Data BOOTCAMP 103.1 GB disk2s3 /dev/disk3 #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Mirror1 *639.8 GB disk3 -> diskutil appleraid list AppleRAID sets (1 found) =============================================================================== Name: Macintosh HD Unique ID: 1953F864-B474-4EB6-8E69-41834EBD0247 Type: Mirror Status: Online Size: 639.8 GB (639791038464 Bytes) Rebuild: manual Device Node: disk3 ------------------------------------------------------------------------------- # Device Node UUID Status ------------------------------------------------------------------------------- 0 disk1s2 25109BAE-5697-40EA-B612-0217851444F7 Online 1 disk0s2 11B83AB0-8148-4DB6-8761-DEF08C855F8D Online =============================================================================== Thanks in advance.

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• Formula to calculate probability of unrecoverable read error during RAID rebuild

  - by OlafM

  I need to compare the reliability of different RAID systems with either consumer or enterprise drives. The formula to have the probability of success of a rebuild, ignoring mechanical problems, is simple: error_probability = 1 - (1-per_bit_error_rate)^bit_read and with 3 TB drives I get 38% probability to experience an URE (unrecoverable read error) for a 2+1 disks RAID5 (4.7% for enterprise drives) 21% for a RAID1 (2.4% for enterprise drives) 51% probability of error during recovery for the 3+1 RAID5 often used by users of SOHO products like Synologys. Most people don't know about this. Calculating the error for single disk tolerance is easy, my question concerns systems tolerant to multiple disks failures (RAID6/Z2, RAIDZ3 and RAID1 with multiple disks). If only the first disk is used for rebuild and the second one is read again from the beginning in case or an URE, then the error probability is the one calculated above squared (14.5% for consumer RAID5 2+1, 4.5% for consumer RAID1 1+2). However, I suppose (at least in ZFS that has full checksums!) that the second parity/available disk is read only where needed, meaning that only few sectors are needed: how many UREs can possibly happen in the first disk? not many, otherwise the error probability for single-disk tolerance systems would skyrocket even more than I calculated. If I'm correct, a second parity disk would practically lower the risk to extremely low values. Am I correct?

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• How to choose the most optimal RAID settings on PE2950

  - by javano

  I have some Dell PowerEdge 2950's with 4x 15k, 150GB Cheetah SAS drives in them. They are going to be VM hosts, CentOS running ESXi with Windows Server 2k8 guests. Some guests will be hosting IIS servers, and others MSSQL servers. I am trying to set the RAID virtual disks settings and can't decide which is more optimal given this situation; Read Policy: Out of Read-Ahead, No-Read-Ahead and Adaptive Read-Ahead, the default is Read-Ahead. I will be making large sequential writes initially, writing out blank images for virtual machine hard drives (lets say 30GBs from /dev/zero for example) so Read-Ahead seems good at first. But within the virtual machines reads could be random from anywhere within their file systems as they are IIS and MSSQL servers, so perhaps No-Read-Ahead is a better idea? Now I think Adaptive Read-Ahead would be better then as a compromise but I don't know much about this option, how does it compare in performance to the others? Write Policy: write-back caching, write-through caching, the default is write-back caching. The default of write-back caching is safer than write-through caching but at a performance expense. My thinking here is that in the event of power loss for example, it seems more likely in my head (this is why I need some clarification!) that damage will occur to a guest VM with write-back caching enabled, so I should favour write-through? I have searched around and there is obviously no definitive answer, so I would like to find out what is best for my situation.

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