On Thu, Mar 17, 2016 at 6:11 AM, Benjamin Coddington <bcodding@xxxxxxxxxx> wrote: > On Thu, 17 Mar 2016, Mkrtchyan, Tigran wrote: >> I agree with Trond, that returning zero bytes without setting eof >> with a high probability a server side issue. We had that situation >> with dCache server, where eof flag was set only if you read beyond >> file size, e.q. READ with count=0 at the offset=file size, we returned >> zero bytes with no eof set. The pynfs test, actually, do retry such >> request and there was an infinite loop. > > But that isn't a short read.. if the request is with count=0 and the > response is 0 without eof, there shouldn't be a retry, and the current > client won't retry in that case, nor fail with EIO. > >> I think, if we (you) add retry on zero byte short-reads >> without eof we may have applications/client hangs in case of >> misbehaving servers. But failing with EIO is not the best >> option. May be it makes sense to query file size in such >> situations? As this is a rare corner case, performance >> penalty will by negligible. > > I do see now that the resistance to this change is because it seems > important to ensure that we make forward progress. This change would lose > that guarantee for a server that gets "stuck". And to Trond's point: if the > zero-length short read really was due to eof at read time, then eof should > have been set. > > It seems tricky for the linux server to determine when to set eof. The best > way might be to adopt the convention that a local short read means we should > set eof and accept the small penalty that a read that completes fully (not > short) up to the end of the file will require another read to detect eof. > That should be a fairly rare case except where file sizes end up being > multiples of rsize. It should be trivial for the linux server to determine when eof is to be set, because it is running in a POSIX environment. As I pointed out earlier, the POSIX spec says that all reads to a regular file that return less than the number of requested bytes are hitting eof. > > Ben > >> >> Tigran. >> >> ----- Original Message ----- >> > From: "Trond Myklebust" <trond.myklebust@xxxxxxxxxxxxxxx> >> > To: "J. Bruce Fields" <bfields@xxxxxxxxxxxx> >> > Cc: "Benjamin Coddington" <bcodding@xxxxxxxxxx>, "Anna Schumaker" <anna.schumaker@xxxxxxxxxx>, "Linux NFS Mailing List" >> > <linux-nfs@xxxxxxxxxxxxxxx> >> > Sent: Wednesday, March 16, 2016 9:02:49 PM >> > Subject: Re: [PATCH] NFS: Retry a zero-length short read >> >> > On Wed, Mar 16, 2016 at 3:56 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: >> >> On Wed, Mar 16, 2016 at 03:46:28PM -0400, Benjamin Coddington wrote: >> >>> So, sounds like fixing this is a good idea on the server. I hope Trond will >> >>> let us know if he still feels that the client ought not to be changed since >> >>> it seems an easy enough fix to avoid a similar problem on another server. >> >>> Perhaps there's a downside I'm not seeing on the client. >> >> >> >> My worry would just be ensuring forward progress--if the client gets >> >> some data back, then at least the next read can start at a later >> >> offset.... With zero reads, we can set a maximum number of retries, I >> >> guess, but that makes it little messy. >> >> >> >>> Or maybe the >> >>> convention of read() returning 0 meaning eof is global enough to cause it to >> >>> be acceptible behavior -- we really should treat a zero-length read response >> >>> without eof as an error. My lack of experience is showing.. :) >> >> >> >> Eh, I think it's legitimately more confusing than it should be. >> >> >> > >> > POSIX is very specific about the cases where you are allowed to return >> > a short read: >> > >> > See http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html >> > >> > "The value returned may be less than nbyte if the number of bytes left >> > in the file is less than nbyte, if the read() request was interrupted >> > by a signal, or if the file is a pipe or FIFO or special file and has >> > fewer than nbyte bytes immediately available for reading. For example, >> > a read() from a file associated with a terminal may return one typed >> > line of data." >> > >> > So I'm guessing most POSIX based server implementations should have no >> > trouble working out exactly when to set the eof flag. However the >> > client has no clue as to what OS your server is based on, which is >> > presumably the main reason why NFS has an eof flag in the first place. >> > -- >> > To unsubscribe from this list: send the line "unsubscribe linux-nfs" in >> > the body of a message to majordomo@xxxxxxxxxxxxxxx >> > More majordomo info at http://vger.kernel.org/majordomo-info.html >> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
