XFS Preallocate using ALLOCSP

[Eric Sandeen]

Smit Shah wrote:
> On 6/16/09, Felix Blyakher <felixb at sgi.com> wrote:

>>> but the write performance is going to suffer.
>> It's not clear why it should. Not doing preallocation doesn't
>> mean that there is no inode updates with every write. Why
>> would extent conversion be more expensive that creating the
>> space (extent) and updating the inode size for every write?
>> It'd interesting to reproduce your results. Any details on
>> your tests and the iometer usage?
> 
> Since fallocate uses the RESVSP cmd for xfs. And as given given for
> RESVSP in man page for xfsctl
> If the  XFS filesystem  is  configured to flag unwritten file extents,
> performance will be negatively affected when writing to preallocated
> space, since extra filesystem transactions are required to convert
> extent  flags  on  the  range  of  the  file  written.

And ext4 must do basically the same thing, as would any fs that flags
unwritten extents.

ext4 may convert more at a time, though, rather than leaving
"fragmented" written/unwritten/written/unwritten regions.

-Eric