Re: question regarding caching

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

On 12/30/2013 09:24 PM, Aastha Mehta wrote:
> Hello,
> 
> I have some questions regarding caching in BTRFS. When a file
> system is unmounted and mounted again, would all the previously
> cached content be removed from the cache after flushing to disk?
> After remounting, would the initial requests always be fetched from
> the disk?
> 
> Rather than a local disk, I have a remote device to which my IO 
> requests are sent and from which the data is fetched. I need
> certain data to be fetched from the remote device after a remount.
> But somehow I do not see any request appearing at the device. I
> even tried to do drop_caches after remounting the file system, but
> that does not seem to help.
> 
> I guess my problem is not related to BTRFS, but since I am working 
> with BTRFS, I wanted to ask here for help. Could any one tell me
> how I can ensure that requests are fetched from the (remote)
> device, especially after file system remount, without having to
> use drop_caches?
> 
> Please let me know if I described the problem too vaguely and
> should give some more details.
The data is probably still cached in the block layer, so after
unmounting, you could try 'echo 1 > /proc/sys/vm/drop_caches' before
mounting again, but make sure to run sync right before doing that,
otherwise you might lose data.