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bug#22768: Crash safety
From: |
Paul Eggert |
Subject: |
bug#22768: Crash safety |
Date: |
Tue, 1 Mar 2016 12:57:05 -0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
On 03/01/2016 10:13 AM, Antonio Diaz Diaz wrote:
[1] http://news.mit.edu/2015/crash-tolerant-data-storage-0824
FSCQ is not even close to ready for prime-time, I'm afraid. Its
prototype is slow compared to conventional file systems (it assumes a
single-threaded kernel, it issues many more writes than ext4 does to
implement a commit, its has been publicly tested only on flash drives,
etc.). The FSQC authors would like to add support for fsync/fdatasync to
get some of that performance back, which seems reasonable -- but at that
point, applications like gzip would still need to call fsync/fdatasync
to avoid losing data.
You may well be right that eventually file system designers will figure
this stuff out so that well-written POSIX applications will not lose
data even if they don't use fsync/fdatasync. However, if FSCQ is any
indication, we're many years away from that. In the meantime
fsync/fdatasync is all we have.
My point is that if gzip has run unsafely for decades without a
reported failure, maybe all those who take these things seriously are
already using file systems safe enough to guarantee that well behaved
tools like gzip do not lose data.
That will be true for many users. Still, I imagine that non-experts
would have a good deal of trouble connecting the dots between lost data
and any gzip invocation that lost the data, and could chalk it up to a
system crash losing data for other reasons. (After all, things are
somewhat chaotic during a crash...) One can find examples on the net
like "How to recover lost/deleted Gzip compressed gz file" (this is for
BYclouder, a commercial tool) that talk about system crashes, and which
indicate (though do not prove) that a real problem exists with gzip.
My sources:
Chen H, Ziegler D, Chajed T, Chlipapa A, Kaashoek MF, Zeldovich N. Using
Crash Hoare logic for certifying the FSCQ file system. SOSP 2015.
https://people.csail.mit.edu/nickolai/papers/chen-fscq.pdf
How to recover lost/deleted Gzip compressed gz file. BYclouder.
2013-04-23.
http://www.byclouder.com/help/recovery/file/archive/how-to-recover-gzip-gz.html