On Tue, Jul 13, 2021 at 5:09 PM Theodore Y. Ts'o <tytso@xxxxxxx> wrote: > > On Tue, Jul 13, 2021 at 12:22:14PM +0530, Shyam Prasad N wrote: > > > > Our team in Microsoft, which works on the Linux SMB3 client kernel > > filesystem has recently been exploring the use of fscache on top of > > ext4 for caching the network filesystem data for some customer > > workloads. > > > > However, the maintainer of fscache (David Howells) recently warned us > > that a few other extent based filesystem developers pointed out a > > theoretical bug in the current implementation of fscache/cachefiles. > > It currently does not maintain a separate metadata for the cached data > > it holds, but instead uses the sparseness of the underlying filesystem > > to track the ranges of the data that is being cached. > > The bug that has been pointed out with this is that the underlying > > filesystems could bridge holes between data ranges with zeroes or > > punch hole in data ranges that contain zeroes. (@David please add if I > > missed something). > > > > David has already begun working on the fix to this by maintaining the > > metadata of the cached ranges in fscache itself. > > However, since it could take some time for this fix to be approved and > > then backported by various distros, I'd like to understand if there is > > a potential problem in using fscache on top of ext4 without the fix. > > If ext4 doesn't do any such optimizations on the data ranges, or has a > > way to disable such optimizations, I think we'll be okay to use the > > older versions of fscache even without the fix mentioned above. > > Yes, the tuning knob you are looking for is: > > What: /sys/fs/ext4/<disk>/extent_max_zeroout_kb > Date: August 2012 > Contact: "Theodore Ts'o" <tytso@xxxxxxx> > Description: > The maximum number of kilobytes which will be zeroed > out in preference to creating a new uninitialized > extent when manipulating an inode's extent tree. Note > that using a larger value will increase the > variability of time necessary to complete a random > write operation (since a 4k random write might turn > into a much larger write due to the zeroout > operation). > > (From Documentation/ABI/testing/sysfs-fs-ext4) > > The basic idea here is that with a random workload, with HDD's, the > cost of writing a 16k random write is not much more than the time to > write a 4k random write; that is, the cost of HDD seeks dominates. > There is also a cost in having a many additional entries in the extent > tree. So if we have a fallocated region, e.g: > > +-------------+---+---+---+----------+---+---+---------+ > ... + Uninit (U) | W | U | W | Uninit | W | U | Written | ... > +-------------+---+---+---+----------+---+---+---------+ > > It's more efficient to have the extent tree look like this > > +-------------+-----------+----------+---+---+---------+ > ... + Uninit (U) | Written | Uninit | W | U | Written | ... > +-------------+-----------+----------+---+---+---------+ > > And just simply write zeros to the first "U" in the above figure. > > The default value of extent_max_zeroout_kb is 32k. This optimization > can be disabled by setting extent_max_zeroout_kb to 0. The downside > of this is a potential degredation of a random write workload (using > for example the fio benchmark program) on that file system. > > Cheers, > > - Ted Hi Ted, Thanks for pointing this out. We'll look into the use of this option. Also, is this parameter also respected when a hole is punched in the middle of an allocated data extent? i.e. is there still a possibility that a punched hole does not translate to splitting the data extent, even when extent_max_zeroout_kb is set to 0? -- Regards, Shyam
