tj wrote:
Bill Davidsen wrote:
tj wrote:
Thomas Jager wrote:
Hi list.
I run a file server on MD raid-5.
If a client reads one big file and at the same time another client
tries to write a file, the thread writing just sits in
uninterruptible sleep until the reader has finished. Only very
small amount of writes get trough while the reader is still working.
I'm having some trouble pinpointing the problem.
It's not consistent either sometimes it works as expected both the
reader and writer gets some transactions. On huge reads I've seen
the writer blocked for 30-40 minutes without any significant writes
happening (Maybe a few megabytes, of several gigs waiting). It
happens with NFS, SMB and FTP, and local with dd. And seems to be
connected to raid-5. This does not happen on block devices without
raid-5. I'm also wondering if it can have anything to do with
loop-aes? I use loop-aes on top of the md, but then again i have
not observed this problem on loop-devices with disk backend. I do
know that loop-aes degrades performance but i didn't think it would
do something like this?
I've seen this problem in 2.6.16-2.6.21
All disks in the array is connected to a controller with a SiI 3114
chip.
I just noticed something else. A couple of slow readers where
running on my raid-5 array. Then i started a copy from another local
disk to the array. Then i got the extremely long wait. I noticed
something in iostat:
avg-cpu: %user %nice %system %iowait %steal %idle
3.90 0.00 48.05 31.93 0.00 16.12
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
....
sdg 0.80 25.55 0.00 128 0
sdh 154.89 632.34 0.00 3168 0
sdi 0.20 12.77 0.00 64 0
sdj 0.40 25.55 0.00 128 0
sdk 0.40 25.55 0.00 128 0
sdl 0.80 25.55 0.00 128 0
sdm 0.80 25.55 0.00 128 0
sdn 0.60 23.95 0.00 120 0
md0 199.20 796.81 0.00 3992 0
All disks are member of the same raid array (md0). One of the disks
has a ton of transactions compared to the other disks. Read
operations as far as i can tell. Why? May be connected with my problem?
Two thoughts on that, if you are doing a lot of directory operations,
it's possible that the inodes being used most are all in one chunk.
Hi thanks for the reply.
It's not directory operations AFAIK. Reading a few files (3 in this
case) and writing one.
The other possibility is that these a journal writes and reflect
updates to the atime. The way to see if this is in some way related
is to mount (remount) with noatime: "mount -o remount,noatime
/dev/md0 /wherever" and retest. If this is journal activity you can
do several things to reduce the problem, which I'll go into (a) if it
seems to be the problem, and (b) if someone else doesn't point you to
an existing document or old post on the topic. Oh, you could also try
mounting the filesystem as etc2, assuming that it's ext3 now. I
wouldn't run that way, but it's useful as a diagnostic tool.
I don't use ext3 i use ReiserFS. ( It seemed like a good idea at the
time. ) It's mounted with -o noatime.
I've done some more testing and i seems like it might be connected to
mount --bind. If i write to a binded mount i get the slow writes. But
if i write directly to the real mount i don't. It might just be a
random occurrence, as the problem always has been inconsistent. Thoughts?
I said I would test, and I did. I don't see a difference with ext3 in
reads at all. I don't see a difference in bind vs. direct for write,
either, but all of my space large enough to have room for a few GB write
had internal bitmaps.
Other info: block size made no consistent difference, changing the
stripe_cache_size helped but was very non-linear in effect, and direct
raid over partitions had the same performance as LVM over raid on other
partitions of the same disks.
Neil: is there a reason (other than ease of coding) why the bitmap isn't
distributed to minimize seeks? ie. put the bitmap for given stripes at
the end of those strips rather than the end of the space.
I have added to my tests-to-do list partitioning a disk such that I have
a small partition and a large, making RAID-10 no bitmap on the small
(multiple drive, obviously) and then RAID-5 on the large, with the
bitmap for RAID-5 on the RAID-10 raw array. The only reason I have any
interest is that I did something like this with JFS, putting the journal
on a dedicated partition with a different chunk size, and it really
helped. If this gives any useful information I'll report, but I'm
building a few analysis tools first, so it will be several weeks.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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