Joerg Schilling writes:
> Roch <[EMAIL PROTECTED]> wrote:
>
> > I would add that this is not a bug or deficientcy in
> > implementation. Any NFS implementation tweak to make 'tar x'
> > go as fast as direct attached will lead to silent data
> > corruption (tar x succeeds but the files don't checksum
> > ok).
> >
> > Interestingly the quality of service of 'tar x' is higher
> > over NFS than direct attach since, with direct attach,
> > there is no guarantee that files are set to storage whereas
> > there is one with NFS.
>
> Why do you beieve this?
>
> Neither Sun tar nor GNU tar call fsync which is the only way to
> enforce data integrity over NFS.
I tend to agree with this although I'd say that in practice,
from performance perspective, calling fsync should be more
relevant for direct attach. For NFS, doesn't close-to-open
and other aspect of the protocol need to enforce much more
synchronous operations ? For tar x over nfs I'd bet the
fsync will be an over-the-wire ops (say 0.5ms) but will not
add an additional I/O latency (5ms) on each file extract.
My target for a single threaded 'tar x' of small files is to
be able to run over NFS at 1 file per I/O latency, no matter
what the backend FS is. I guess that 'star -yes-fsync' over
direct attach should behave the same ? Or do you have
concurrency in there...see below.
>
> If you like to test this, use star. Star by default calls
> fsync before it closes a written file in x mode. To switch this
> off, use star -no-fsync.
>
> > Net Net, for single threaded 'tar x', data integrity
> > consideration forces NFS to provide a high quality slow
> > service. For direct attach, we don't have those data
> > integrity issues, and the community has managed to get by the
> > lower quality higher speed service.
>
> What dou you have in mind?
>
> A tar that calls fsync in detached threads?
>
You tell me ? We have 2 issues
can we make 'tar x' over direct attach, safe (fsync)
and posix compliant while staying close to current
performance characteristics ? In other words do we
have the posix leeway to extract files in parallel ?
For NFS, can we make 'tar x' fast and reliable while
keeping a principle of least surprise for users on
this non-posix FS.
-r
> Jörg
>
> --
> EMail:[EMAIL PROTECTED] (home) Jörg Schilling D-13353 Berlin
> [EMAIL PROTECTED] (uni)
> [EMAIL PROTECTED] (work) Blog: http://schily.blogspot.com/
> URL: http://cdrecord.berlios.de/old/private/
> ftp://ftp.berlios.de/pub/schily
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