Re: VFAT truncate performance

[Bodo Eggert <7eggert@elstempel.de>]

Mattias Rönnblom <hofors@lysator.liu.se> wrote:

> extending files by ftruncate(2) runs very slow on VFAT file
> systems. On my USB harddisk w/ VFAT, it takes 14 seconds to extend an
> empty file to 1 GB. On a memory stick, it takes well over 4 minutes.
> 
> My question is: is this problem on the conceptual level (ie there is
> no way of extending files on FAT that doesn't involve many disk
> operations) or is the current Linux fs driver suboptimal in this
> respect?

Linux needs to zero files it truncate-extends because of security guarantees.

You could temporarily ignore the truncate after create if it's followed by
writing the file (defer untill first non-write), but it will be a BAD hack.
It might work.

Default: open(O_WRITE|O_CREAT|O_TRUNC) -> do it, goto State 1
         otherwise -> just do it

State 1: ftruncate -> remember offset instead of executing ftruncate,
                      goto State 2
         otherwise -> goto Default

State 2: write     -> do it, stay in State 2 unless file size increases
                      beyond fake size, then goto Default
         stat      -> return fake size
         otherwise -> really do ftruncate, goto Default

It might cause some operations to be slow you'd expect to be fast, and
I'm not sure how it has to deal with concurrent access.
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