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redgsturbo
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Joined: 24 Jun 2005
Posts: 283
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 Posted: Tue Oct 02, 2007 4:54 pm Post subject: Software RAID5 question?? |
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| How many drives MUST I have for software RAID5? I know the answer is 3 for it to be worthwhile, but I know you can create expandable arrays and create arrays with one drive missing (with no protection in that scenario). What I want to do is create a 2 disk array in raid 5 of 750G drives (logically speaking: one data drive, one parity drive), and grow that to an eventual 4 or 6 as $$ allows and space requires. Can this be done? I just want the system up and useful now, without having to cough up $1k in drives just yet |
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lagalopex
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Joined: 16 Oct 2004
Posts: 565
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| Posted: Tue Oct 02, 2007 5:15 pm Post subject: |
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Just build the raid5 with two disks.
When you have more disks, you can grow the array. Just take care that you need to resize the fs after that  |
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razze
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Joined: 09 Mar 2005
Posts: 161
Location: Espoo, Finland
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| Posted: Tue Oct 02, 2007 5:28 pm Post subject: |
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Hello!
You can create the array at the moment, but it will not be fully functional but work in degraded mode. The performance will probably not be as good as with at least three disks, but the most important: you will NOT have any redundancy at all, as the one drive that can fail is already missing! You will, however, have the same amount of available disk space, so once you add a third drive, the array will be fully functional.
Basically you just create an array with three disk with one of them missing.
<fixed typos>
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redgsturbo
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Joined: 24 Jun 2005
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| Posted: Tue Oct 02, 2007 5:59 pm Post subject: |
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| razze wrote: | Hello!
You can create the array at the moment, but it will not be fully functional but work in degraded mode. The performance will probably not be as good as with at least three disks, but the most important: you will NOT have any redundancy at all, as the one drive that can fail is already missing! You will, however, have the same amount of available disk space, so once you add a third drive, the array will be fully functional.
Basically you just create an array with three disk with one of them missing.
<fixed typos> |
this would only be the case if i started with a 3 drive array of which only 2 were present... i was wondering if i can start with a 2 drive array of 2 drives |
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Cyker
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Joined: 15 Jun 2006
Posts: 1746
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| Posted: Tue Oct 02, 2007 9:19 pm Post subject: |
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*Technically*, you MUST have a minimum of 3 drives for RAID5.
RAID5 is a bit like RAID0, but with a parity block, so you need 2 disks for the data, and a third for the parity block minimum.
This allows a single drive to fail without you loosing any data.
You *can* do it with two, but this is a big hack because you're essentially making a degraded RAID5 array, i.e. one that has already lost a disk.
In this mode, you've essentially got a really slow 2-disk RAID0 array (i.e. the worst of both worlds; Slowness of RAID5 and non-redundancy of RAID0)
Now, you *can* do this, and pray that your array doesn't crash until you can afford a third disk, but at that point you won't gain any data space, just get the redundancy back incase one of the disks fails.
My opinion: Don't do it.
If you want speed and don't care about redundancy, use RAID0 and convert to RAID5 after.
If you want data security, use RAID1 and convert to RAID5 after.
(Assuming you can convert after... anyone want to weigh in on that? NB: Copying all the data off and then rebuilding the array is *NOT* converting!) |
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redgsturbo
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Joined: 24 Jun 2005
Posts: 283
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| Posted: Tue Oct 02, 2007 11:45 pm Post subject: |
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| Cyker wrote: | *Technically*, you MUST have a minimum of 3 drives for RAID5.
RAID5 is a bit like RAID0, but with a parity block, so you need 2 disks for the data, and a third for the parity block minimum.
This allows a single drive to fail without you loosing any data.
You *can* do it with two, but this is a big hack because you're essentially making a degraded RAID5 array, i.e. one that has already lost a disk.
In this mode, you've essentially got a really slow 2-disk RAID0 array (i.e. the worst of both worlds; Slowness of RAID5 and non-redundancy of RAID0)
Now, you *can* do this, and pray that your array doesn't crash until you can afford a third disk, but at that point you won't gain any data space, just get the redundancy back incase one of the disks fails.
My opinion: Don't do it.
If you want speed and don't care about redundancy, use RAID0 and convert to RAID5 after.
If you want data security, use RAID1 and convert to RAID5 after.
(Assuming you can convert after... anyone want to weigh in on that? NB: Copying all the data off and then rebuilding the array is *NOT* converting!) |
I understand how all the raid levels work... but what I'm wondering is if the parity block can't be calculated from ONE other block (yes... stupid, but may allow seamless integration of more drives later) |
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Cyker
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Joined: 15 Jun 2006
Posts: 1746
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| Posted: Wed Oct 03, 2007 7:27 am Post subject: |
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Mmm... an intriguing question... It'd be sortof like RAID1, except both drives are different...
I don't think mdadm supports such a thing however  ; I /believe/ that it can only just create a degraded array (1 disk's worth of half-data, the other 1-disk's worth of parity which is calculated to make up the rest of the half-data)...
I could be wrong 'tho |
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