XFS Preallocate using ALLOCSP

[Felix Blyakher]

On Jun 16, 2009, at 5:05 PM, Eric Sandeen wrote:

> Smit Shah wrote:
>>
>> 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.

Converting more at a time would require writing extra zeros
to extra space converted, but not covered with write. Again,
it's a trade off, and there is no clear winner. Well, at least
not in my mind, until proven that some operations are more
efficient than others.

Thanks,
Felix