Re: XFS and "hole".

[Russell Cattelan <cattelan@xxxxxxx>]

Vadim wrote:

> XFS supports "holes". But we created file 100mb with 0  only in it, and it's actual size 
> was 100mb, for zeros this support doesn't work. We created other file wrote some bytes at the 
> beggining then did seek ~1GB and wrote to the end couple more bytes. XFS was showing it's size of 
> 1GB, but actual size of it on the disc was only few bytes, that's called "HOLE" as I 
> understand. But it doesn't work for 0s, if you write a big ammount of zeros. What we have to do, to 
> write an add-on that would work for zeros, but problemm is we can't find any good enough 
> specification. We could dig over whole source that was being written several years, and VERY badly 
> documentated, but we are limited in time... If anybody could help with specification or give an 
> advice, we would really apretiate it!
>
> Thank you.
> Vadim Vohmjanin.
>
> -----------------------------------------
> ITV - Sinu lemmiksaated internetis!
> http://www.itv.ee
>
>  
The filesystem does do any sort of:  is this block all 0's so I'll skip 
writting it and create a hole.
That would be a large amount of overhead as the content of every block 
would have to be scanned
before written to disk to make that determination.

Holes are created by seeking around a file a writing a few bytes 
wherever you happen to
be.

Basically it is up to the app do determine it a set of data is just 0's 
and skip the write.

-Russell Cattelan