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Re: Problem about very high Average Read/Write Request Time

To: Peter Grandi <pg@xxxxxxxxxxxxxxxxxxx>, Linux fs XFS <xfs@xxxxxxxxxxx>
Subject: Re: Problem about very high Average Read/Write Request Time
From: Bernd Schubert <bernd.schubert@xxxxxxxxxxx>
Date: Mon, 20 Oct 2014 10:00:34 +0200
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References: <CALSoAzD4ccHXBuD6mT3ggqMf1j_kDEK-RNMOeRLq+N+NiWVQXg@xxxxxxxxxxxxxx> <20141018143848.3baf3266@xxxxxxxxxxxxxx> <21571.36364.518119.806191@xxxxxxxxxxxxxxxxxx>
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On 10/19/2014 12:10 PM, Peter Grandi wrote:
>>> I am using xfs on a raid 5 (~100TB) and put log on external
>>> ssd device, the mount information is: /dev/sdc on
>>> /data/fhgfs/fhgfs_storage type xfs
>>> (rw,relatime,attr2,delaylog,logdev=/dev/sdb1,sunit=512,swidth=15872,noquota).
>>> when doing only reading / only writing , the speed is very
>>> fast(~1.5G), but when do both the speed is very slow (100M),
>>> and high r_await(160) and w_await(200000).
> 
>> What are your kernel version, mount options and xfs_info output ?
> 
> Those are usually important details, but in this case the
> information that matters is already present.
> 
> There is a ratio of 31 (thirty one) between 'swidth' and 'sunit'
> and assuming that this reflects the geometry of the RAID5 set
> and given commonly available disk sizes it can be guessed that
> with amazing "bravery" someone has configured a RAID5 out of 32
> (thirty two) high capacity/low IOPS 3TB drives, or something
> similar.
> 
>   It is even "braver" than that: if the device name
>   "/data/fhgfs/fhgfs_storage" is dedscriptive, this "brave"
>   RAID5 set is supposed to hold the object storage layer of a
>   BeeFS highly parallel filesystem, and therefore will likely
>   have mostly-random accesses.
> 

Where do you get the assumption from that FhGFS/BeeGFS is going to do
random reads/writes or the application of top of it is going to do that?


Bernd

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