linux-kernel - Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

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Message-ID: <CAH2r5mvM=OX+F4uqMh_c24DXLxwgRX2UzX3bsYqM00JPzbtjqA@mail.gmail.com>
Date:	Wed, 27 Feb 2013 16:40:10 -0600
From:	Steve French <smfrench@...il.com>
To:	Dave Chiluk <dave.chiluk@...onical.com>
Cc:	Jeff Layton <jlayton@...ba.org>,
	"Stefan (metze) Metzmacher" <metze@...ba.org>,
	Dave Chiluk <chiluk@...onical.com>,
	Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

On Wed, Feb 27, 2013 at 4:24 PM, Dave Chiluk <dave.chiluk@...onical.com> wrote:
> On 02/27/2013 10:34 AM, Jeff Layton wrote:
>> On Wed, 27 Feb 2013 12:06:14 +0100
>> "Stefan (metze) Metzmacher" <metze@...ba.org> wrote:
>>
>>> Hi Dave,
>>>
>>>> When messages are currently in queue awaiting a response, decrease amount of
>>>> time before attempting cifs_reconnect to SMB_MAX_RTT = 10 seconds. The current
>>>> wait time before attempting to reconnect is currently 2*SMB_ECHO_INTERVAL(120
>>>> seconds) since the last response was recieved.  This does not take into account
>>>> the fact that messages waiting for a response should be serviced within a
>>>> reasonable round trip time.
>>>
>>> Wouldn't that mean that the client will disconnect a good connection,
>>> if the server doesn't response within 10 seconds?
>>> Reads and Writes can take longer than 10 seconds...
>>>
>>
>> Where does this magic value of 10s come from? Note that a slow server
>> can take *minutes* to respond to writes that are long past the EOF.
> It comes from the desire to decrease the reconnection delay to something
> better than a random number between 60 and 120 seconds.  I am not
> committed to this number, and it is open for discussion.  Additionally
> if you look closely at the logic it's not 10 seconds per request, but
> actually when requests have been in flight for more than 10 seconds make
> sure we've heard from the server in the last 10 seconds.
>
> Can you explain more fully your use case of writes that are long past
> the EOF?  Perhaps with a test-case or script that I can test?  As far as
> I know writes long past EOF will just result in a sparse file, and
> return in a reasonable round trip time *(that's at least what I'm seeing
> with my testing).  dd if=/dev/zero of=/mnt/cifs/a bs=1M count=100
> seek=100000, starts receiving responses from the server in about .05
> seconds with subsequent responses following at roughly .002-.01 second
> intervals.  This is well within my 10 second value.

Note that not all Linux file systems support sparse files and
certainly there are cifs servers running on operating systems other
than Linux which have popular file systems which don't support sparse
files (e.g. FAT32 but there are many others) - in any case, writes
after end of file can take a LONG time if sparse files are not
supported and I don't know a good way for the client to know that
attribute of the server file system ahead of time (although we could
attempt to set the sparse flag, servers can and do lie)


-- 
Thanks,

Steve
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