On 22/02/2011 14:38, Stan Hoeppner wrote:
David Brown put forth on 2/22/2011 2:57 AM:
On 21/02/2011 22:51, Stan Hoeppner wrote:
RAID5/6 have decent single streaming read performance, but sub optimal
random read, less than sub optimal streaming write, and abysmal random
write performance. They exhibit poor random read performance with high
client counts when compared to RAID0 or RAID10. Additionally, with an
analysis "cluster" designed for overall high utilization (no idle
nodes), one node will be uploading data sets while others are doing
analysis. Thus you end up with a mixed simultaneous random read and
streaming write workload on the server. RAID10 will give many times the
throughput in this case compared to RAID5/6, which will bog down rapidly
under such a workload.
I'm a little confused here. It's easy to see why RAID5/6 have very poor
random write performance - you need at least two reads and two writes
for a single write access. It's also easy to see that streaming reads
will be good, as you can read from most of the disks in parallel.
However, I can't see that streaming writes would be so bad - you have to
write slightly more than for a RAID0 write, since you have the parity
data too, but the parity is calculated in advance without the need of
any reads, and all the writes are in parallel. So you get the streamed
write performance of n-[12] disks. Contrast this with RAID10 where you
have to write out all data twice - you get the performance of n/2 disks.
I also cannot see why random reads would be bad - I would expect that to
be of similar speed to a RAID0 setup. The only exception would be if
you've got atime enabled, and each random read was also causing a small
write - then it would be terrible.
Or am I missing something here?
I misspoke. What I meant to say is RAID5/6 have decent streaming and
random read performance, less than optimal *degraded* streaming and
random read performance. The reason for this is that with one drive
down, each stripe for which that dead drive contained data and not
parity the stripe must be reconstructed with a parity calculation when read.
That makes lots of sense - I was missing the missing word "degraded"!
I don't think the degraded streaming reads will be too bad - after all,
you are reading the full stripe anyway, and the data reconstruction will
be fast on a modern cpu. But random reads will be very bad. For
example, if you have 4+1 drives in a RAID5, then one in every 5 random
reads will be on the dead drive, and will require 4 reads. That means
random reads will take 180% of the normal time, or almost half the
performance.
This is another huge advantage RAID 10 has over the parity RAIDs: zero
performance loss while degraded. The other two big ones are vastly
lower rebuild times and still very good performance during a rebuild
operation as only two drives in the array take an extra hit from the
rebuild: the survivor of the mirror pair and the spare being written.
Yes, this is definitely true - RAID10 is less affected by running
degraded, and recovering is faster and involves less disk wear. The
disadvantage compared to RAID6 is, of course, if the other half of a
disk pair dies during recovery then your raid is gone - with RAID6 you
have better worst-case redundancy.
Once md raid has support for bad block lists, hot replace, and non-sync
lists, then the differences will be far less clear. If a disk in a RAID
5/6 set has a few failures (rather than dying completely), then it will
run as normal except when bad blocks are accessed. This means for all
but the few bad blocks, the degraded performance will be full speed.
And if you use "hot replace" to replace the partially failed drive, the
rebuild will have almost exactly the same characteristics as RAID10
rebuilds - apart from the bad blocks, which must be recovered by parity
calculations, you have a straight disk-to-disk copy.
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