On Wed, 1 Jul 2015 11:53:07 -0300 Roberto Spadim
[off-list ref] wrote:

i could do write-behind in a already up md device?
i'm trying echo writebehind > /sys/block/md0/md/dev-sdc1 and it return error

/sys/block/md0/md/dev-sdc1# echo write_behind >state
-bash: echo: write error: Invalid argument

You really should be using mdadm, not trying to poke directly at sysfs.

The "Write behind" functionality involves two separate configurations.

1/ the write-intent bitmap must enable write behind - it records a
   maximum number of outstanding "behind" requests".  To arrange this
   you remove any bitmap that you have and add a correctly configured
   one.

2/ individual devices can be marked as "write mostly", in which case the
   writes to those devices are "behind" writes.  you can enable this
   by writing "writemostly" to the 'state' file.


NeilBrown

2015-06-25 0:28 GMT-03:00 Roberto Spadim [off-list ref]:

quoted

nice :)  i read about it some years ago and i was trying to remember, i will
test, thanks neil!

2015-06-24 19:59 GMT-03:00 NeilBrown [off-list ref]:

quoted

On Tue, 23 Jun 2015 12:01:33 -0300 Roberto Spadim
[off-list ref] wrote:

quoted

hi guys, i'm with a doubt
when i use witemostly, how the md handle the slower drive
(writemostly) for example i have 2 ssd that support 500mb/s write
running raid1, i included a hdd with +- 100mb/s write with writemostly
flag
when i execute a dd if=/dev/zero of=./test , i get near to 200mb/s
with dd, and using iostat -d 1 -k, i get +- 100mb/s
there's some kind of configuration to writemostly drive? something
like 'buffer size' or someting like it?

You need to also enable --write-behind if you want md/raid1 to not be
slowed down so much by a slower device.
However this only really helps hide latency.  There is nothing that can
be done to help over-all throughput.

When you set --write-behind you can provide a number which is the
number of outstanding requests that can be in flight to the
write-mostly device.

If your writes are "bursty" in nature - so lots of writes together,
then longer pauses with no writes, then this write-behind buffer can
hide the delay caused by the slower device.  If you write continuously,
then there is no way that the slower device can keep up and the
over-all throughput will be reduced to match the slowest device.

This functionality was originally written for cases where both devices
had the same throughput, but one was higher latency, due to being
several kilometres away on the other end of an optical fibre.

NeilBrown

--
Roberto Spadim
SPAEmpresarial - Software ERP
Eng. Automação e Controle

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