Re: write-behind has no measurable effect?

[Roberto Spadim <roberto@spadim.com.br>]

at raid1.c there´s a example with ssd and hd with different speeds

2011/2/15 Roberto Spadim <roberto@spadim.com.br>:
> writes will be the speed of slowest mirror (ssd or hd)
> read can have speed improvement with this patch
>
> there´s some options
>
> /sys/block/md0/md/read_balance_mode
>
> near_head = today implementation
> round_robin => could be usefull if you have ssd only, since
> round_robin consider that access time is the same for any drive (hard
> disk access time is diferent for random and sequencial)
> stripe => i didn´t get good benchmarks, but it´s nice to have it,
> since we could put it over network, some sectors on one disk others on
> another (nbd)
> time_based => here you should send me some information about you disk
> for example:
>    access time of mirrors (check your drive information website, ssd
> are normaly <0.1ms, hard disks near 10~20 ms)
>    sequencial read speed (use dd if=/dev/sda of=/dev/zero bs=4096,
> change the block size to size you will use with your filesystem, for
> vertex2 ssd i´m using bs=4096, for disks it´s a good value too, since
> disks bs ~= number of heads (2,4,8) )
>
> send me the access time and sequencial read speed and i make the
> values to tune your sysfs (/sys/block/md0/md/read_balance_config)
>
> you will need to :
>
> echo "time_based" > /sys/block/md0/md/read_balance_mode
> echo "disks informations" > /sys/block/md0/md/read_balance_config
>
> for each mirror, you can´t use sysfs file to configure it, maybe a
> bash script is a better solution to configure it, on future version i
> will change it and put at /sys/block/md0/md/dev-xxxx/
>
> 2011/2/15 Andras Korn <korn@raidlist.elan.rulez.org>:
>> On Tue, Feb 15, 2011 at 06:10:17AM -0300, Roberto Spadim wrote:
>>
>>> andras could you make some benchmarks to raid1 with round robin read balance?
>>> at this site:
>>> www.spadim.com.br/raid1/
>>>
>>> it's kernel 2.6.37 based
>>
>> Yes, I can do that. Can you give me some hints on what specific
>> configuration to try? I see you have some sysfs tunables. My raid1 array
>> consists of two spinning disks and an SSD, all local.
>>
>> Do you expect this patch to make a difference in my case? With the spinning
>> disks marked as write-mostly, I'm getting close to the read performance of
>> the SSD (except for very small random reads, for some reason).
>>
>> It's random writes that are much slower than with only the SSD.
>>
>> --
>>                     Andras Korn <korn at elan.rulez.org>
>>                  There is no spoon(). But there is a fork().
>> --
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>
>
>
> --
> Roberto Spadim
> Spadim Technology / SPAEmpresarial
>



-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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