Re: O_DIRECT to md raid 6 is slow

[Stan Hoeppner <stan@hardwarefreak.com>]

On 8/16/2012 4:50 PM, Miquel van Smoorenburg wrote:
> On 16-08-12 1:05 PM, Stan Hoeppner wrote:
>> On 8/15/2012 6:07 PM, Miquel van Smoorenburg wrote:
>>> Ehrm no. If you modify, say, a 4K block on a RAID5 array, you just have
>>> to read that 4K block, and the corresponding 4K block on the
>>> parity drive, recalculate parity, and write back 4K of data and 4K
>>> of parity. (read|read) modify (write|write). You do not have to
>>> do I/O in chunksize, ehm, chunks, and you do not have to rmw all disks.
>>
>> See:  http://www.spinics.net/lists/xfs/msg12627.html
>>
>> Dave usually knows what he's talking about, and I didn't see Neil nor
>> anyone else correcting him on his description of md RMW behavior.
> 
> Well he's wrong, or you're interpreting it incorrectly.
> 
> I did a simple test:
> 
> * created a 1G partition on 3 seperate disks
> * created a md raid5 array with 512K chunksize:
>   mdadm -C /dev/md0 -l 5 -c $((1024*512)) -n 3 /dev/sdb1 /dev/sdc1
> /dev/sdd1
> * ran disk monitoring using 'iostat -k 5 /dev/sdb1 /dev/sdc1 /dev/sdd1'
> * wrote a single 4K block:
>   dd if=/dev/zero bs=4K count=1 oflag=direct seek=30 of=/dev/md0
> 
> Output from iostat over the period in which the 4K write was done. Look
> at kB read and kB written:
> 
> Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
> sdb1              0.60         0.00         1.60          0          8
> sdc1              0.60         0.80         0.80          4          4
> sdd1              0.60         0.00         1.60          0          8
> 
> As you can see, a single 4K read, and a few writes. You see a few blocks
> more written that you'd expect because the superblock is updated too.

I'm no dd expert, but this looks like you're simply writing a 4KB block
to a new stripe, using an offset, but not to an existing stripe, as the
array is in a virgin state.  So it doesn't appear this test is going to
trigger RMW.  Don't you need now need to do another write in the same
stripe to to trigger RMW?  Maybe I'm just reading this wrong.

-- 
Stan