On Sat, 3 Nov 2012, Ian Lepore wrote:
On Sat, 2012-11-03 at 17:06 -0500, Adam Vande More wrote:
On Sat, Nov 3, 2012 at 4:30 PM, Brett Glass <br...@lariat.net> wrote:
Have been following the thread related to SU+J, and am wondering: why is it
considered to be undesirable on SSDs (assuming that they have good wear
leveling)?
Superstition
Yeah, that's what it must be. Or... it could be well-informed choice.
Journaling increases the number of writes. That puts wear on any disk,
mechanical or SSD, and it takes time. What it buys you is better
performance if you get into a crash recovery situation. It's perfectly
reasonable for someone to make the decision that their SSD can finish an
fsck so fast that there's no point in paying any penalty for the extra
writes for journaling.
The journal entries are 32 bytes per in SUJ. So the number of extra
writes is down in the noise. The journaling also gets you asynchronous
partial truncation and a few other asynchronous operations that are sync
in SU. It does cost slightly more cpu time and more memory. I'm not
saying you're making the wrong choice. I'm just saying that it's not
clear that you should or should not use it with SSDs.
I have a 256G SSD here with about 200G of data on it, and fsck without
journaling takes about 3 minutes. I can live with that. With more data
or a slower drive I might make a different choice.
If you are happy with 3 minutes this is very reasonable and I assume you
turn off bg fsck.
Jeff
-- Ian
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