Hi all, I suffered a lot for the terrific performance of aptitude on a BTRFS filesystem. I don't think that BTRFS is a slow filesystem, but it seems that some aptitude (or dpkg) patterns are capable to highlight the btrfs slowness in some corner case.
In order to alleviate this problem, I wrote a small script which calls
aptitude with the LD_PRELOAD libeatmyadata library. And now I want to share
this idea (which is not my own, you can see several suggestion about that in
the net) in order to collect suggestions and/or critics.
For who which don't know what is libeatmydata, the package info says:
"This package contains a small LD_PRELOAD library (libeatmydata) and a couple
of helper utilities designed to transparently disable fsync and friends (like
open(O_SYNC)). This has two side-effects: making software that writes data
safely to disk a lot quicker and making this software no longer crash
safe...."
In order to reduce the risk of data loss, my script create a snapshot of the
root filesystem before calling the aptitude. After the end of aptitude a double
call to sync, flush all the data on the disk. Finally the snapshot is removed.
The root of my filesystem is a btrfs subvolume (not the real root one).
Inspired by ArchLinux I called it "__active", and it is placed on the real
root of the btrfs filesystem. In order to manage the subvolume and its
snapshots I mount the real root of the btrfs filesystem under /var/btrfs
Before running aptitude my script creates a snapshot of "__active" subvolume
called "__rollback", and put it in the root of the btrfs filesystem.
During the boot I have two entry in the grub menu, the first one starts the
system using as root the "__active" subvolume. The other one starts the system
using the "__rollback" subvolume.
The idea is that if something goes wrong before calling the sync command, I
can safely start the system from a coherent (i.e. no half package
installation) system.
The next step is to automatically start from a "__rollback" subvolume if
present (remember that the subvolume "__rollback" is automatically removed
after the two sync) during the execution of the initramfs. Another idea is to
maintain the last 3-4-5...N rollbacks in order to be able to go back to old
configuration.
Now, during a packages update the performances with btrfs are comparable to
the ext4 ones. Obviously what I says about dpkg/aptitude, should be true also
for other packages manager (like urpm/yum/rpm, pacman....)
Of course there are some caveats:
a) the filesystem must use only one subvolume (the snapshotting is not
recursive)
b) if a package [re]starts a daemon (like a DB), it inherited the LD_PRELOAD
environment variable, so the daemon has the sync disabled
For the second point I am looking a way to enable the libeatmydata only for
the dpkg program. I found that it is possible to pass --force-unsafe-io to
dpkg in order to reduce the sync calls. But it seems that some sync are anyway
performed.
Comments are welcome.
BR
G.Baroncelli
-------------------------------------------
$ cat btrfs-aptitude
#!/bin/bash
ROLLBACK=/var/btrfs/__rollback
ACTIVE=/var/btrfs/__active
if [ ! -d "$ACTIVE" ]; then
echo "Cannot find $ACTIVE"
exit
fi
if [ -d "$ROLLBACK" ]; then
echo "Found $ROLLBACK; remove it"
btrfs subvolume delete "$ROLLBACK" || exit
fi
echo "Snap shot $ACTIVE -> $ROLLBACK"
btrfs subvolume snapshot "$ACTIVE" "$ROLLBACK" || exit
sync
(
export LD_PRELOAD=/usr/lib/libeatmydata/libeatmydata.so
aptitude
)
echo "Syncing..."
sync
sleep 5s
sync
echo "Remove $ROLLBACK"
btrfs subvolume delete "$ROLLBACK"
------------------------------------------------------------
--
gpg key@ keyserver.linux.it: Goffredo Baroncelli (ghigo) <kreij...@inwind.it>
Key fingerprint = 4769 7E51 5293 D36C 814E C054 BF04 F161 3DC5 0512
signature.asc
Description: This is a digitally signed message part.
