I'm testing a few systems that attempt to log data to disk reliably. I
bought a brand new Intel SSD (X25-M G2) for this purpose. It appears to
me that this disk does *not* store data reliably when there are power
failures, even with write barriers, even with the cache disabled. I'm
surprised that this disk might be this broken (possible), but it may
also mean I've made a mistake. Is there any possible way that I have a
bug in the test described below? The test works as expected with a
couple SATA magnetic disks.
Configuration:
* Linux 2.6.32 (a distributed with Ubuntu 10.04)
* SATA SSD directly attached to the system's built-in controller (Intel
N10/ICH7)
* ext4 with default options (meaning barrier=1)
* Disable the write cache (hdparm -W 0 /dev/sdb)
The test:
1. Write a 64 MB file of zeros (first use fallocate, then zero fill)
2. fsync()
3. write() blocks of this file with a sequence number.
4. fdatasync()
5. Send UDP packet reporting the sequence number written.
6. Go to 3.
While this test is running, I pull the power out of the drive to
simulate a hard failure. On the magnetic disks I have, this works as
expected: On reboot, the log file contains the complete record that was
reported as last written (it may also contain part of the next record).
On the X25-M, when I use large writes (128 kB), it loses data fairly
frequently (every couple attempts): I either see the last log record as
being before the reported one, or occasionally I get a media error when
reading back the file.
I'm surprised that this disk could be this broken, but I suppose it is
possible. Any help is welcomed. Thanks,
Evan Jones
--
Evan Jones
http://evanjones.ca/
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