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Re: mkfs.xfs error creating large agcount an raid

To: linux-xfs@xxxxxxxxxxx
Subject: Re: mkfs.xfs error creating large agcount an raid
From: Marcus Pereira <marcus@xxxxxxxxxxx>
Date: Sun, 26 Jun 2011 02:53:43 -0300
In-reply-to: <4E0694CC.8050003@xxxxxxxxxxxxxxxxx>
References: <4E063BC6.9000801@xxxxxxxxxxx> <4E0694CC.8050003@xxxxxxxxxxxxxxxxx>
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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
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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