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Re: [PATCH] xfs_io: fix random pread/pwrite to honor offset

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH] xfs_io: fix random pread/pwrite to honor offset
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Wed, 09 Apr 2014 15:55:09 -0500
Cc: xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5345B209.5040906@xxxxxxx>
References: <53459C4E.9030902@xxxxxxxxxxx> <5345B209.5040906@xxxxxxx>
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.4.0
On 4/9/14, 3:48 PM, Mark Tinguely wrote:
> On 04/09/14 14:15, Eric Sandeen wrote:
>> xfs_io's pread & pwrite claim to support a random IO mode
>> where it will do random IOs between offset & offset+len.
>>
>> However, offset was ignored, and we did the IOs between 0
>> and len instead.
>>
>> Clang caught this by pointing out that the calculated/normalized
>> "offset" variable was never read.
>>
>> (NB: If the range is larger than RAND_MAX, these functions don't
>> work, but that's always been true, so I'll leave it for another
>> day...)
>>
>> Signed-off-by: Eric Sandeen<sandeen@xxxxxxxxxx>
>> ---
>>
>> I haven't tested this w/ xfstests but I don't see anyone who calls
>> pread/pwrite with "-R" (and it's not documented in the manpage!)
>>
>> diff --git a/io/pread.c b/io/pread.c
>> index a42baed..465c22b 100644
>> --- a/io/pread.c
>> +++ b/io/pread.c
>> @@ -246,7 +246,7 @@ read_random(
> 
> offset can be the end of file offset. Do we care that the read (/write)
> could start or end past the end of file?

It now behaves the same as a normal forward read does if you tell it
to go past EOF:

xfs_io> truncate 1g
xfs_io> pread 1g 1m
read 0/1048576 bytes at offset 1073741824
0.000000 bytes, 0 ops; 0.0000 sec (0.000000 bytes/sec and 0.0000 ops/sec)
xfs_io> pread -R 1g 1m
read 0/1048576 bytes at offset 1073741824
0.000000 bytes, 0 ops; 0.0000 sec (0.000000 bytes/sec and 0.0000 ops/sec)

so I think it's ok?

>>
>>       *total = 0;
>>       while (count > 0) {
>> -        off = ((random() % range) / buffersize) * buffersize;
>> +        off = ((offset + (random() % range)) / buffersize) * buffersize;
>>           bytes = do_pread(fd, off, buffersize, buffersize);
>>           if (bytes == 0)
>>               break;
> 
> Looks like this was introduced in:
> commit 8fb2237e65555ff540e8b6108ffccfffefe239ac
> Author: Nathan Scott <nathans@xxxxxxx>
> Date:   Fri Nov 11 14:25:18 2005 +0000
> 
> Provide further debugging options and tweaks for analysing the read/write 
> paths.
> Merge of master-melb:xfs-cmds:24372a by kenmcd.
> 
>  ---
> 
> If it was broken for 8.5 years, I think it could be removed.

Eh, could, or we could fix it.  :)  I suppose this means it needs a test... :/

-Eric

> --Mark.
> 

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