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Talk:Sugar on a Stick/Stick Layout (view source)
Revision as of 08:30, 12 March 2010
, 08:30, 12 March 2010livecd Digest, Vol 59, Issue 10 Message: 3
livecd Digest, Vol 59, Issue 10
Message: 3
Date: Fri, 12 Mar 2010 08:23:46 +0000
From: James Heather <j.heather@surrey.ac.uk>
Subject: [Fedora-livecd-list] Write cacheing on a live USB stick (was:
Preventing access to /mnt/live (inter alia))
To: "livecd@lists.fedoraproject.org" <livecd@lists.fedoraproject.org>
Message-ID: <1268382226.32238.53.camel@oscar.bluehouse.dyndns.org>
Content-Type: text/plain; charset="utf-8"
Time to update you on my little experiments with write cacheing on a
live USB stick.
It does look as though I'd misunderstood what the settings
in /proc/sys/vm/ are for. I'd taken it that these just controlled paging
of virtual memory (hence the 'vm'), i.e., writing to swap. But it seems
that they also control the filesystem write cache. So I have stuck with
ext4, and I now have in my kickstart:
%post
cat >> /etc/rc.d/rc.local << EOF
#more aggressive page writing in case of crash or power cut
echo 500 > /proc/sys/vm/dirty_expire_centisecs
EOF
%end
which means that nothing is allowed to remain in the cache for more than
5 sec. (Strictly speaking, it is marked as dirty after 5 sec, and then
gets written at the next wakeup, which happens every 5 sec by default;
so in fact it can stay in the cache for up to 10 sec.)
Experiments suggest that this works nicely. I just wrote something like
for ((;;)); do date; sleep 1; done | tee logfile
and then yanked the USB stick out and rebooted it to see how much had
been written; it wasn't ever more than a few seconds behind the terminal
window. The LED also flashed briefly every 5-10 sec, which is what you'd
hope.
This does seem more efficient than forcibly syncing every few seconds,
which will work, but will also block disk reads during the sync process.
It also means that the benefits of delayed allocation don't get seen.
Let's just hope nothing crashes during the real exam and it's all
irrelevant!
James
Message: 3
Date: Fri, 12 Mar 2010 08:23:46 +0000
From: James Heather <j.heather@surrey.ac.uk>
Subject: [Fedora-livecd-list] Write cacheing on a live USB stick (was:
Preventing access to /mnt/live (inter alia))
To: "livecd@lists.fedoraproject.org" <livecd@lists.fedoraproject.org>
Message-ID: <1268382226.32238.53.camel@oscar.bluehouse.dyndns.org>
Content-Type: text/plain; charset="utf-8"
Time to update you on my little experiments with write cacheing on a
live USB stick.
It does look as though I'd misunderstood what the settings
in /proc/sys/vm/ are for. I'd taken it that these just controlled paging
of virtual memory (hence the 'vm'), i.e., writing to swap. But it seems
that they also control the filesystem write cache. So I have stuck with
ext4, and I now have in my kickstart:
%post
cat >> /etc/rc.d/rc.local << EOF
#more aggressive page writing in case of crash or power cut
echo 500 > /proc/sys/vm/dirty_expire_centisecs
EOF
%end
which means that nothing is allowed to remain in the cache for more than
5 sec. (Strictly speaking, it is marked as dirty after 5 sec, and then
gets written at the next wakeup, which happens every 5 sec by default;
so in fact it can stay in the cache for up to 10 sec.)
Experiments suggest that this works nicely. I just wrote something like
for ((;;)); do date; sleep 1; done | tee logfile
and then yanked the USB stick out and rebooted it to see how much had
been written; it wasn't ever more than a few seconds behind the terminal
window. The LED also flashed briefly every 5-10 sec, which is what you'd
hope.
This does seem more efficient than forcibly syncing every few seconds,
which will work, but will also block disk reads during the sync process.
It also means that the benefits of delayed allocation don't get seen.
Let's just hope nothing crashes during the real exam and it's all
irrelevant!
James