On 27/07/16 10:24, Adam Goryachev wrote:
Hi all,
I know, age old question, but I have the chance to change things up a
bit, and I wanted to collect some thoughts/ideas.
Currently I am using 8 x 480GB Intel SSD in a RAID5, then LVM on top,
DRBD on top, and finally iSCSI on top (and then used as VM raw disks for
mostly windows VM's).
Wow. More layers than a wedding cake.
My suspicion is that the actual load is made up of rather small random
read/write, because that is the scenario that produced the worst
performance results when I was initially setting this up, and seems to
be what we are getting in practice.
The last option is, what if I moved to RAID10? Would that provide a
significant performance boost (completely removes the need to worry
about chunk/stripe size because we always just write the exact data we
want, no need to read/compute/write)?
OR, is that read/compute overhead negligible since I'm using SSD and
read performance is so quick?
I'll only comment from personal experience with anecdotal evidence.
I have a RAID10 comprised of 6 256GB SSD (3 Intel & 3 Samsung) used as
the backing for multiple VMs (raw files on Ext4).
Initially I played with a number of RAID types when setting up the array
(back in 2012) and found RAID10 offered the best compromise for my use
case. This was based on CPU usage (raid 5 & 6 parity calculations on
*every write*), the need for RMW cycles for small writes and trying to
balance block sizes. None of these things are an issue with RAID10 and
in general I found a measurable reduction in overhead and consequential
performance boost in the VM's.
This is a single machine with a relatively underpowered AMD FX-8350 CPU.
I found that with multiple VM's hitting the disks I was losing enough
CPU time to RAID overhead that it made things noticeably less responsive.
My only issue is half the disks return a deterministic value after
discard and the other half don't, so any raid check operations return a
gazillion mismatches. Not an operational issue, but one worth mentioning
if you were to use different model drives.
Regards,
Brad
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