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Re: Disappointing performance of copy (MD raid + XFS)

To: xfs@xxxxxxxxxxx
Subject: Re: Disappointing performance of copy (MD raid + XFS)
From: Asdo <asdo@xxxxxxxxxxxxx>
Date: Thu, 10 Dec 2009 02:16:35 +0100
Cc: linux-raid <linux-raid@xxxxxxxxxxxxxxx>
In-reply-to: <4B204783.7040109@xxxxxxxxxxxxx>
References: <4B204334.1000605@xxxxxxxxxxxxx> <4B204783.7040109@xxxxxxxxxxxxx>
User-agent: Thunderbird (X11/20090608)
Asdo wrote:
Asdo wrote:
and I think I have seen around 10MB/sec when they are of 500KB (this transfer at 10MB/sec was in parallel with another faster one however).
Yes I definitely confirm: right now I have just 1 rsync copy running, it's in a zone where files are around 500KB on average, and it's going at 9 MB/sec. Stack traces of the writer process conform to what I have posted in my previous email, even now that the writer is the only process using the destination array.
Excuse me, I am going nuts...
In this case of 9MB/sec for 500KB files, stack traces on the writer are indeed very similar to what I have posted, but the relative frequency of the two type of stack traces is different: 20%: waiting on the reader (this almost never happened when using multiple parallel rsyncs)
50%: xlog_state_get_iclog_space+0xed/0x2d0
30%: xfs_buf_lock+0x1e/0x60

The reader is waiting either on select (on the writer I guess) or on this:
 [<ffffffff810da74d>] sync_page+0x3d/0x50
 [<ffffffff810da769>] sync_page_killable+0x9/0x40
 [<ffffffff810da682>] __lock_page_killable+0x62/0x70
[<ffffffff810db8be>] T.768+0x1ee/0x440 [<ffffffff810dbbc6>] generic_file_aio_read+0xb6/0x1d0 [<ffffffffa031cd95>] xfs_read+0x115/0x2a0 [xfs] [<ffffffffa031832b>] xfs_file_aio_read+0x5b/0x70 [xfs] [<ffffffff8111ec32>] do_sync_read+0xf2/0x130 [<ffffffff8111f215>] vfs_read+0xb5/0x1a0 [<ffffffff8111f81c>] sys_read+0x4c/0x80 [<ffffffff81012002>] system_call_fastpath+0x16/0x1b [<ffffffffffffffff>] 0xffffffffffffffff

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