I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
On 8/13/05, Ulrik S. Kofod usk@cybersite.dk wrote:
I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
I have seen some RAID -1 setups, but why 3 drives?
In Raid - 1 Setup, if one drive fails, all others fail. So what is the point in using 3 drives in RAID-1?
I am not an expert on this. But this is quite interesting. Can you clarify this a bit for me please?
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duffmckagan ha scritto:
On 8/13/05, Ulrik S. Kofod usk@cybersite.dk wrote:
I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
I have seen some RAID -1 setups, but why 3 drives?
In Raid - 1 Setup, if one drive fails, all others fail. So what is the point in using 3 drives in RAID-1?
I am not an expert on this. But this is quite interesting. Can you clarify this a bit for me please?
with 3 disk you can use a raid 5, 2 disk for data striping with 1 disk for distributed parity.
if you are a raid's beginner you must read and search document with google...
just for say, i post one link to adaptec web site (that document explaine the raid technology)
http://www.adaptec.com/pdfs/ACSP_RAID_Ch4.pdf
Ulrik S. Kofod wrote:
I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
I really don't understand the question. RAID-1 requires an even number of disks since disk(s) A simply is a mirror image of disk(s) B. So with 3 disks you could have a 2-drive mirror and designate the 3rd drive as a hot spare. So if one of your mirrored drives fails, the hot spare takes over. You could just as easily do a 3-disk RAID-5 array. It would be substantially faster on reads than a 2-disk mirror. It could be faster or slower on writes depending on which hardware RAID card you're using or how fast your processor is if you're doing software RAID.
Cheers,
On Saturday 13 August 2005 07:55, Ulrik S. Kofod wrote:
I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
I believe you can do either, but setting it up as a spare works ok, too. Nice when one drive of a mirrored RAID1 starts throwing errors to throw the spare on, let the sync happen, then hot fail the drive that's throwing errors. For IDE of course you will have to power down to pull the drive; depending upon your setup you might have hotswap (SATA can do hotswap, but the software support is poor; SCSI can do hotswap as long as it's either SCA or a special hotswap carrier (I have three such carriers for 68-pin UW-SCSI that has a bus isolator board that allows hotswapping regular UW-SCSI drives)) and then it's easy, particularly with some SCSI controllers, where you can stop a device and then rescan/start a device on the fly.
I just migrated a single drive LVM setup to a RAID 1 mirror last week. The procedure isn't terribly hard, but it revolved around setting up a single-drive RAID 1 (requires the --force switch to mdadm), creating the physical volume, volume group(s), and logical volumes, then copying all data from the old single drive to the new drive. Once the copy was verified, I simply zeroed the partition table of the old drive, used fdisk to create a software raid partition (the entire disk in this case), then used mdadm to hotadd the old drive and make it active (mdadm --grow is the part that made it active).
For RAID 1 the number of drives is not limited to 2, contrary to common belief. You can have three or more; just understand that a RAID 1 set with 4 250GB drives (for example) will present itself as a single 250GB RAID (with three mirrors) and not as a 500GB RAID (for that, you want RAID 1+0 or a RAID 1 of LVM volume groups; the RAID layer would see two RAID devices, /dev/md0 and /dev/md1, for example, and the volume group would span the two RAID devices (/dev/md0 and /dev/md1, again), which is a little more robust and much more flexible than a classic RAID 1+0).
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
The md layer will have to perform three writes instead of two; the drive interface technology will impact the performance of a triple write more than anything else, with straight IDE taking probably a 33% performance hit (having 66% of the writes-per-second of the two-drive RAID), with well-designed SCSI setups (multiple host adapter channels) potentially taking a small fraction of that hit.
Lamar Owen sagde:
On Saturday 13 August 2005 07:55, Ulrik S. Kofod wrote:
I have 3 disks that I want to setup in a raid 1, my questions is should I setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the time?
I believe you can do either, but setting it up as a spare works ok, too. Nice when one drive of a mirrored RAID1 starts throwing errors to throw the spare on, let the sync happen, then hot fail the drive that's throwing errors.
I just thought the it would be safer to mirror all 3 from the start as if one fails the system will rely on only one disk until the spare is synced.
For RAID 1 the number of drives is not limited to 2, contrary to common belief. You can have three or more; just understand that a RAID 1 set with 4 250GB drives (for example) will present itself as a single 250GB RAID
I know I can add as many drives as there is room for in the box to a RAID1 and that the available disk space will be equal to the size of smallest disk (if they are not the same size). I just cant see why I should add one as a spare, when I can mirror it, and then no sync is needed when one fails (until I replace it).
Space is not an issue, the drives has more than enough space, so I would rather go for a RAID1 than a RAID5, where I know the system will work on just a single drive. Performance is also a minor concern as the box is pretty much oversized for the job.
Will it perform *much* better to only mirror two and have the 3rd as a spare. Is it not safer to mirror all 3 drives all the time ?
The md layer will have to perform three writes instead of two; the drive interface technology will impact the performance of a triple write more than anything else, with straight IDE taking probably a 33% performance hit (having 66% of the writes-per-second of the two-drive RAID), with well-designed SCSI setups (multiple host adapter channels) potentially taking a small fraction of that hit.
I was thinking of putting only one disk on each controller (the MB controller and 2 PCI controllers). Safety is my main concern, and if one controller fails it will only take down 1 disk. Is that overkill? Would it be better to have just one per channel i.e. two on the MB controlller and 1 on a PCI controller?
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Chris Mauritz -
duffmckagan -
Lamar Owen -
ML -
Ulrik S. Kofod