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 2015-06-25 0:28 GMT-03:00 Roberto Spadim <roberto@xxxxxxxxxxxxx>: > 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 <neilb@xxxxxxxx>: >> >> On Tue, 23 Jun 2015 12:01:33 -0300 Roberto Spadim >> <roberto@xxxxxxxxxxxxx> wrote: >> >> > 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 -- Roberto Spadim SPAEmpresarial - Software ERP Eng. Automação e Controle -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
