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zidarsk8

Member

Registered: 2011-12-06

Posts: 97

Bcache with btrfs with writetback is slow

Hi,

setup: 2* 180GB ssd, 2*3TB hdd

the ssd drives have swap and bcache on them, bcache is on raid0 on the ssd drives,

md0 = raid 0 on ssd1 and ssd2

bcache0 = hdd1 with md0 bcache
bcache1 = hdd2 with md0 bcache

root = btrfs on raid1 bcache0 and bcache1

bcache is mounted as writethrough so I was thinking that write speed should be faster than the hdd. but

So why am I getting these kind of results

$ time dd if=/dev/zero of=deleteme bs=1M count=5k
5120+0 records in
5120+0 records out
5368709120 bytes (5.4 GB) copied, 4.40479 s, 1.2 GB/s

real	0m24.003s
user	0m0.010s
sys	0m0.057s

also is my setup wrong or bad?

ps: here is exactly how I've set up my system http://pastebin.com/ZwwZMd40

Last edited by zidarsk8 (2014-03-09 03:05:41)

yhylord

Member

Registered: 2013-12-24

Posts: 13

Re: Bcache with btrfs with writetback is slow

I don't think it's very slow...

zidarsk8

Member

Registered: 2011-12-06

Posts: 97

Re: Bcache with btrfs with writetback is slow

I don't think it's very slow...

it says it's fast, but compare these two numers

4.40479 s,
and
real    0m24.003s

so it says it takes only 4.4 seconts, but I have to wait at least 24 seconds for it to finish. thats  6 times slower than it says it should be.