Ext3 Stride/Stripe-Width Calculation for 3-Disk Software RAID1E

Question

For reasons too complex to get into here, I'm using a 3-disk RAID1E on some linux systems. See here for more info on that.

So my understanding is that for optimizing filesystem layout on this volume, you use the following calculation:

chunk-size = 64kB
block-size = 4kB
stride = chunk / block = 16
stripe-width = stride * ( (numdisks)/2 ) = 16 * 1.5 = 24

However, mkfs.ext3 gives a warning when I use that calculation for setting the stripe-width, that it should be a power of two.

So my question is, am I doing it right? Should I be treating it like a standard four-disk RAID10, since the stripes are of the same size?

Update: it's not a degraded array, it's a fully supported configuration. Read the link from the first paragraph.

3 disk...RAID 10? You mean you're running an intentionally degraded array? — MDMarra, Jul 20 '12 at 18:29
If you had the correct number of drives in your array then it would work out correctly. — user9517, Jul 20 '12 at 18:32
@MDMarra - no, it's not degraded - linux md supports 3-disk RAID10 where each stripe is mirrored across two disks, per the link in the first paragraph. — Alex G, Jul 20 '12 at 18:39
It appears that Linux MD is lying to you. What you have is a R1E. The calculations you have is for an R10 which requires an even number of disks. — user9517, Jul 20 '12 at 18:48

score 2 · Answer 1 · answered Jul 20 '12 at 18:38

2

Setting your stripe width higher than 64kB will be suboptimal.

Any write of a size higher than 64kB will result in 4 writes - one write to one disk, one write to another, and two writes to one.

Just set your stripe width to 64kB.

answered Jul 20 '12 at 18:38

MikeyB

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Why? What's actually happening with the disk stripes and writes that makes this the case? – Alex G Jul 20 '12 at 20:13

score 0 · Answer 2 · answered Aug 19 '20 at 19:23

You are starting your optimisation from the wrong place. You need to start from the I/O size you are optimising for (e.g. block size that your application does reads/writes in), and optimise the whole stack for that. I wrote an article on this exact subject that explains why storage stack and file system alignment is important, which you may find helpful.

score 0 · Accepted Answer · answered Aug 09 '12 at 20:52

I ran some experiments using XFS instead of ext3 on various size and levels of MD RAID. It seems like the following formula applies across the board:

# chunk is specified in mdadm create, block in mkfs
stride = chunk / block
stripe-width = ((slices - parity) * stride

Where parity is zero for RAID0/10/1E, one for RAID5, two for RAID6.

So in the case of my original question, stripe-width should be set to 48 (64kB chunk, 4kB block, 3 slices, zero parity). When I use these settings, mkfs.ext3 no longer gives a warning about stripe-width not being divisible by the stride.

Ext3 Stride/Stripe-Width Calculation for 3-Disk Software RAID1E

3 Answers3

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