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Re: very slow file deletion on an SSD

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: very slow file deletion on an SSD
From: Joe Landman <joe.landman@xxxxxxxxx>
Date: Sun, 27 May 2012 13:14:41 -0400
Cc: Krzysztof Adamski <k@xxxxxxxxxxx>, Stefan Ring <stefanrin@xxxxxxxxx>, linux-raid <linux-raid@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
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On 05/27/2012 12:07 PM, Eric Sandeen wrote:
On 5/27/12 9:59 AM, joe.landman@xxxxxxxxx wrote:
This is going to be a very fragmented file.  I am guessing that this
is the reason for the long duration delete.   I'll do some more
measurements before going to 3.4.x as per Eric's note.

filefrag -v should also tell you how many fragments, and because it
uses fiemap it probably won't run into the same problems.

But it sounds like we can just assume very high fragmentation.

[root@siFlash test]# filefrag  1.r.48.0
1.r.48.0: 1364 extents found

It's not addressing the exact issue, but why are the files so fragmented?
Are they very hole-y or is it just an issue with how they are written?
Perhaps preallocation would help you here?

Possibly. We are testing the system using fio, and doing random reads and writes. I'll see if we can do a preallocation scheme (before/during) for the files.

So to summarize, the delete performance will be (at least) in part a function of the fragmentation? A directory full of massively fragmented files will take longer to delete than a directory of contiguous and larger extents? And I did some experimentation using xfs_repair, and it seems to be the case there as well ... the higher level of fragmentation, the longer the repair seems to take.


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