Gentoo Archives: gentoo-user

From: William Kenworthy <billk@×××××××××.au>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] Btrfs: how to fix slow sequential write performance?
Date: Wed, 25 Sep 2013 09:22:33
Message-Id: 5242AB3E.1030505@iinet.net.au
In Reply to: [gentoo-user] Btrfs: how to fix slow sequential write performance? by "Marc Stürmer"
1 On 25/09/13 16:54, Marc Stürmer wrote:
2 > Greetings fellow Gentooistas,
3 >
4 > at the moment I am doing some testing and evaluating of Btrfs on my own
5 > system. Why? Just because I can and I am curious about it.
6 >
7 > Because I am doing that for evaluation purpose please don't give advise
8 > like "go ZFS" or something like that, thank you.
9 >
10 > Kernel is 3.10.7, the file system is on one hard drive only (SATA).
11 >
12 > And here comes my question: sequential write access seems to be slow as
13 > hell, actually something around like 10 Mbyte/s according to dd's output.
14 >
15 > The same HDD performed under ext4 with around 90 Mbyte/s.
16 >
17 > Command is something like: dd if=/dev/zero of=file.img bs=1G count=150
18 >
19 > Nothing else is running at the time of writing, no scrub/balance on the
20 > file system or other process eating up much cpu time.
21 >
22 > Mount option is "relatime" only. So is this a well known issue only with
23 > this kernel or generally with this file system at the moment?
24 >
25 > Are there any possible fixes to squeeze better performance out of it or
26 > is this unlikely to happen so that I should better take that into
27 > account of my evaluation and maybe dump it then?
28 >
29 > Thanks in advance.
30 >
31
32 How old is the fs? - I am ran into a major slowdown due to COW snapshot
33 fragmentation (VMs on ceph using btrfs) - a known problem. Using that
34 kernel is also giving me an eventual hard lockup if I try and do a
35 recursive defrag (on another system), but the ssd on an apple air
36 defraged fine.
37
38 btrfs is also seems very sensitive to how full it is. Have you tried a
39 btrfsck? - some errors seem to cause slowdowns as well.
40
41 BillK