On 06/28/2013 09:25 AM, Martin wrote:
On 28/06/13 16:39, Hugo Mills wrote:
On Fri, Jun 28, 2013 at 11:34:18AM -0400, Josef Bacik wrote:
On Fri, Jun 28, 2013 at 02:59:45PM +0100, Martin wrote:
On kernel 3.8.13:
Using two equal performance SATAII HDDs, formatted for btrfs
raid1 for both data and metadata and:
The second disk appears to suffer about x8 the read activity of
the first disk. This causes the second disk to quickly get
maxed out whilst the first disk remains almost idle.
Total writes to the two disks is equal.
This is noticeable for example when running "emerge --sync" or
running compiles on Gentoo.
Is this a known feature/problem or worth looking/checking
further?
So we balance based on pids, so if you have one process that's
doing a lot of work it will tend to be stuck on one disk, which
is why you are seeing that kind of imbalance. Thanks,
The other scenario is if the sequence of processes executed to do
each compilation step happens to be an even number, then the
heavy-duty file-reading parts will always hit the same parity of
PID number. If each tool has, say, a small wrapper around it, then
the wrappers will all run as (say) odd PIDs, and the tools
themselves will run as even pids...
Ouch! Good find...
To just test with a:
for a in {1..4} ; do ( dd if=/dev/zero of=$a bs=10M count=100 & ) ; done
ps shows:
martin 9776 9.6 0.1 18740 10904 pts/2 D 17:15 0:00 dd
martin 9778 8.5 0.1 18740 10904 pts/2 D 17:15 0:00 dd
martin 9780 8.5 0.1 18740 10904 pts/2 D 17:15 0:00 dd
martin 9782 9.5 0.1 18740 10904 pts/2 D 17:15 0:00 dd
More to the story from atop looks to be:
One disk maxed out with x3 dd on one cpu core, the second disk
utilised by one dd on the second CPU core...
Looks like using a simple round-robin is pathological for an even
number of disks, or indeed if you have a mix of disks with different
capabilities. File access will pile up on the slowest of the disks or
on whatever HDD coincides with the process (pid) creation multiple...
So... an immediate work-around is to go all SSD or work in odd
multiples of HDDs?!
Rather than that: Any easy tweaks available please?
Thanks,
Martin
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Interesting discussion. I just put up Gkrellm here to look at this
issue. What I am seeing is perhaps disturbing. I have my root file
system as RAID 1 on two drives, /dev/sda and /dev/sdb. I am seeing
continual read and write activity on /dev/sdb, but nothing at all on
/dev/sda. I am sure it will eventually do a big write on /dev/sda to
sync, but it appears to be essentially using one drive in normal
routine. All my other filesystems, /usr, /var, /opt, are RAID 1 across
five drives. In this case all drives are active in use ... except the
fifth drive. I actually observed a long flow of continual reads and
writes very balanced across the first four drives in this set and then,
like a big burp, a huge write on the fifth drive. But absolutely no
reads from the fifth drive so far. Very interesting behavior? These are
all SATA ncq configured drives. The first pair are notebook drives, the
five drive set are all seagate 2.5" enterprise level drives. - George
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