[linux-lvm] fsync() and LVM

Marco Colombo linux-lvm at esiway.net
Fri Mar 13 17:46:33 UTC 2009

Hi, I'm a long time user of both PostgreSQL and LVM. So far I've been quite
happy with both. But a recent thread on the PostgreSQL list made me
unconfortable. What is this thing they're referring to, fsync()'s being
ignored? Makes me feel like I'm running on thin ice, without even
knowing. Before I start phasing out LVM from all my PostgreSQL installations
(as they suggest), I'd like to hear some kind of confirmation.
This is quite scary.


In my understanding:

fsync():  force data from OS memory to disk (ending up in the disk cache)
write barrier: force data from disk cache to disk platters

If you disable write-back cache on the disks, you no longer need write
barriers. But apparently they claim LVM being unsafe even with disks
caches in write-thru mode, which surprises me a lot.


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