[Lustre-discuss] short writes

[Andrew Perepechko]

Hello John,

please note that the Lustre client does not write data to a disk device,
rather it sends and receives data through network, particularly, through a
socket in case of ksocklnd. Even if the call may not be considered
slow with respect to indefiniteness, it still is slow as compared to the
"fast I/O" (local disk I/O). I can recall several requests of adding
even more interruption points made by people who were looking for the support
of premature read/write termination (and, I believe, l_wait_event usage in
1.8 is such that since a network request has been sent, the read/write
system call cannot be interrupted until the corresponding lustre timeout
has happened or a reply has been received)

Best wishes,
Andrew.

>On 07/08/2010 08:53 AM, Kevin Van Maren wrote:
>It also says "The issue of which files or file types are interruptible 
>is considered an implementation design issue. This is often affected 
>primarily by hardware and reliability issues."
>
>For Linux, the signal(7) manpage indicates that read(2), readv(2), 
>write(2), writev(2), and ioctl(2) calls on "slow" devices should return 
>-EINTR when interrupted by a signal, and goes on to say that "slow" 
>devices are ones "where the I/O call may block for an indefinite time, 
>for example, a terminal, pipe, or socket.  (A disk is not a slow device 
>according to this definition.)"
>
>Nowhere does it say something really helpfully clear like "Writing to a 
>regular file shall suspend the calling process until such time as..." 
>But, I interpret this to mean that operations on regular files are not 
>interruptible, and should not return -EINTR.  Moreover, I understand 
>that this is the consensus among those unlucky enough to care.
>
>On the other hand, there are some explicitly specified situations which 
>will result in short writes to a regular file, like file size limits.

-- 
John L. Hammond, Ph.D.
ICES, The University of Texas at Austin
jhammond at ices.utexas.edu <http://lists.lustre.org/mailman/listinfo/lustre-discuss>
(512) 471-9304

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