Re: raid5 to utilize upto 8 cores
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On 16/08/2012 07:58, Stan Hoeppner wrote:
On 8/15/2012 9:56 PM, vincent Ferrer wrote:- My storage server has upto 8 cores running linux kernel 126.96.36.199. - I created a raid5 device of 10 SSDs . - It seems I only have single raid5 kernel thread, limiting my WRITE throughput to single cpu core/thread.The single write threads of md/RAID5/6/10 are being addressed by patches in development. Read the list archives for progress/status. There were 3 posts to the list today regarding the RAID5 patch.Question : What are my options to make my raid5 thread use all the CPU cores ? My SSDs can do much more but single raid5 thread from mdadm is becoming the bottleneck. To overcome above single-thread-raid5 limitation (for now) I re-configured. 1) I partitioned all my 10 SSDs into 8 partitions: 2) I created 8 raid5 threads. Each raid5 thread having partition from each of the 8 SSDs 3) My WRITE performance quadrupled because I have 8 RAID5 threads. Question: Is this workaround a normal practice or may give me maintenance problems later on.No it is not normal practice. I 'preach' against it regularly when I see OPs doing it. It's quite insane. The glaring maintenance problem is that when one SSD fails, and at least one will, you'll have 8 arrays to rebuild vs one. This may be acceptable to you, but not to the general population. With rust drives, and real workloads, it tends to hammer the drive heads prodigiously, increasing latency and killing performance, and decreasing drive life. That's not an issue with SSD, but multiple rebuilds is. That and simply keeping track of 80 partitions.
The rebuilds will, I believe, be done sequentially rather than in parallel. And each rebuild will take 1/8 of the time a full array rebuild would have done. So it really should not be much more time or wear-and-tear for a rebuild of this monster setup, compared to a single raid5 array rebuild. (With hard disks, it would be worse due to head seeks - but still not as bad as you imply, if I am right about the rebuilds being done sequentially.)
However, there was a recent thread here about someone with a similar setup (on hard disks) who had a failure during such a rebuild and had lots of trouble. That makes me sceptical to this sort of multiple array setup (in addition to Stan's other points).
And of course, all Stan's other points about maintenance, updates to later kernels with multiple raid5 threads, etc., still stand.
There are a couple of sane things you can do today to address your problem: 1. Create a RAID50, a layered md/RAID0 over two 5 SSD md/RAID5 arrays. This will double your threads and your IOPS. It won't be as fast as your Frankenstein setup and you'll lose one SSD of capacity to additional parity. However, it's sane, stable, doubles your performance, and you have only one array to rebuild after an SSD failure. Any filesystem will work well with it, including XFS if aligned properly. It gives you an easy upgrade path-- as soon as the threaded patches hit, a simple kernel upgrade will give your two RAID5 arrays the extra threads, so you're simply out one SSD of capacity. You won't need to, and probably won't want to rebuild the entire thing after the patch. With the Frankenstein setup you'll be destroying and rebuilding arrays. And if these are consumer grade SSDs, you're much better off having two drives worth of redundancy anyway, so a RAID50 makes good sense all around. 2. Make 5 md/RAID1 mirrors and concatenate them with md/RAID linear. You'll get one md write thread per RAID1 device utilizing 5 cores in parallel. The linear driver doesn't use threads, but passes offsets to the block layer, allowing infinite core scaling. Format the linear device with XFS and mount with inode64. XFS has been fully threaded for 15 years. Its allocation group design along with the inode64 allocator allows near linear parallel scaling across a concatenated device, assuming your workload/directory layout is designed for parallel file throughput. #2, with a parallel write workload, may be competitive with your Frankenstein setup in both IOPS and throughput, even with 3 fewer RAID threads and 4 fewer SSD "spindles". It will outrun the RAID50 setup like it's standing still. You'll lose half your capacity to redundancy as with RAID10, but you'll have 5 write threads for md/RAID1, one per SSD pair. One core should be plenty to drive a single SSD mirror, with plenty of cycles to spare for actual applications, while sparing 3 cores for apps as well. You'll get unlimited core scaling with both md/linear and XFS. This setup will yield the best balance of IOPS and throughput performance for the amount of cycles burned on IO, compared to Frankenstein and the RAID50.
For those that don't want to use XFS, or won't have balanced directories in their filesystem, or want greater throughput of larger files (rather than greater average throughput of multiple parallel accesses), you can also take your 5 raid1 mirror pairs and combine them with raid0. You should get similar scaling (the cpu does not limit raid0). For some applications (such as mail server, /home mount, etc.), the XFS over a linear concatenation is probably unbeatable. But for others (such as serving large media files), a raid0 over raid1 pairs could well be better. As always, it depends on your load - and you need to test with realistic loads or at least realistic simulations.
 If you are one of the uneducated masses who believe dd gives an accurate measure of storage performance, then ignore option #2. Such a belief would indicate you thoroughly lack understanding of storage workloads, and thus you will be greatly disappointed with the dd numbers this configuration will give you.
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