Re: [squid-users] aufs lseek on read()

On Wed, 29 Sep 2004, Michael Puckett wrote:

> I have been examining both the aufs and ufs versions of squid with truss
> and have seen that the async i/o version has thousands more calls to
> lseek than the non async version. On looking at the source of
> squidaio_do_read() it does indeed do a lseek() followed by a read()
> where the non async read doesn't seem to. This leads to 2 questions:
>
> 1. According to the man page entry, threaded implementations should not
> use lseek() followed by read() as atomicity between the calls cannot be
> guaranteed. It recommends using pread() instead. Is this correct?

Correct. But in Squid there is an guarantee there is only one call at a
time per filedescriptor so it is not a problem.

> 2. Why is the lseek() necessary in the async read method but not the
> blocking read method?

Because the async-io lowlevel interface was modelled along the lines of
POSIX AIO where the read() operation is read(file, buffer, buffersize,
file_offset). This was perhaps not the best design, but reasonable.

Regards
Henrik
Received on Thu Sep 30 2004 - 05:50:04 MDT