Re: mkfs.xfs error creating large agcount an raid

[Marcus Pereira <marcus@task.com.br>]

Em 25-06-2011 23:09, Stan Hoeppner escreveu:
> On 6/25/2011 2:49 PM, Marcus Pereira wrote:
>> I have an issue when creating xfs volume using large agcounts on raid
>> volumes.
> Yes, you do have an issue, but not the one you think.
Ok, but seems something that should be corrected. Isn't that?

>> /dev/md0 is a 4 disks raid 0 array:
>>
>> ----------------------------------------
>> # mkfs.xfs -V
>> mkfs.xfs version 3.1.4
>>
>> # mkfs.xfs -d agcount=1872 -b size=4096 /dev/md0 -f
> mkfs.xfs queries mdraid for its parameters and creates close to the
> optimal number of AGs, sets the stripe width, etc, all automatically.
> The default number of AGs for striped mdraid devices is 16 IIRC, and
> even that is probably a tad too high for a 4 spindle stripe.  Four or
> eight AGs would probably be better here, depending on your workload,
> which you did not state.  Please state your target workload.
The system is a heavy loaded email server.
> At 1872 you have 117 times the number of default AGs.  The two main
> downsides to doing this are:
The default agcount was 32 at this system.
> 1. Abysmal performance due to excessive head seeking on an epic scale
> 2. Premature drive failure due to head actuator failure
There is already insane head seeking at this server, hundreds of 
simultaneous users reading their mailboxes. In fact I was trying to 
reduce the head seeking with larger agcounts.

> Now, the above assumes your "4 disks" are mechanical drives.  If these
> are actually SSDs then the hardware won't suffer failures, but
> performance will likely be far less than optimal.
The 4 disks are mechanical, in fact each of them are 2 SCSI HD raid 1 
hardware raid 0 array but the OS sees it as a single device.
So its a raid 10 with hardware raid 1 and software raid 0.

> Why are you attempting to create an insane number of allocation groups?
>   What benefit do you expect to gain from doing so?
>
> Regardless of your answer, the correct answer is that such high AG
> counts only have downsides, and zero upside.
It is still a test to find an optimal agcount, there are several of this 
servers and each of them would be with a different agcount. I was trying 
to build an even larger agcount something like 20000 to 30000. :-)
The goal is to try to keep less or even 1 mailboxes per AG so more 
sequential reading at each mailbox access and less random seek at the 
volume. I dont know if it was going to work like I was thinking.
I got this idea at this post and was giving it a try: 
http://www.techforce.com.br/news/linux_blog/lvm_raid_xfs_ext3_tuning_for_small_files_parallel_i_o_on_debian

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