28 Sep
2017
28 Sep
'17
6:53 p.m.
From: Theodore Ts'o
when a file was truncated and then rewritten, and
"truncate this file"
packet got reordered and got received after the "here's the new 4k of
contents of the file", Hilar[i]ty Enused.
This sounds _exactly_ like a bad bug found in the RVD protocol (Remote Virtual
Disk - a simple block device emulator). Disks kept suffering bit rot (damage
to the inodes, IIRC). After much suffering, and pain trying to debug it (lots
of disk writes, how do you figure out the one that's the problem), it was
finally found (IIRC, it wasn't something thinking about it, they actually
caught it). Turned out (I'm pretty sure my memory of the bug is correct), if
you had two writes of the same block in quick sucession, and the second was
lost, if the first one's ack was delayed Just Enough...
They had an unused 'hint' (I think) field in the protocol, and so they
recycled that to be a sequence number, so they could tell ack's apart.
Noel
28 Sep
28 Sep
8:09 p.m.
New subject: RFS was: Re: UNIX of choice these days?
On Thu, Sep 28, 2017 at 08:53:54AM -0400, Noel Chiappa wrote:
At least at Project Athena, we only used RVD as a way to do read-only
dissemination of system software, since RVD couldn't provide any kind
of file sharing, which was important at Athena. So we used NFS
initially, and later on, transitioned to the Andrew File System (AFS).
The fact that with AFS you could do live migration of a user's home
directory, in the middle of day, to replace a drive that was making
early signs of failure --- was such that all of our ops people
infinitely preferred AFS to NFS. NFS meant coming in late at night,
after scheduling downtime, to replace a failing disk, where as with
AFS, aside from a 30 second hiccup when the AFS volume was swept from
one server to another, was such that AFS was referred to as "crack
cocaine". One ops people had a sniff of AFS, they never wanted to go
back to NFS.
- Ted
From: Theodore
Ts'o
when a file was truncated and then rewritten, and
"truncate this file"
packet got reordered and got received after the "here's the new 4k of
contents of the file", Hilar[i]ty Enused.
This sounds _exactly_ like a bad bug found in the RVD protocol (Remote Virtual
Disk - a simple block device emulator). Disks kept suffering bit rot (damage
to the inodes, IIRC). After much suffering, and pain trying to debug it (lots
of disk writes, how do you figure out the one that's the problem), it was
finally found (IIRC, it wasn't something thinking about it, they actually
caught it). Turned out (I'm pretty sure my memory of the bug is correct), if
you had two writes of the same block in quick sucession, and the second was
lost, if the first one's ack was delayed Just Enough... 
8:35 p.m.
New subject: RFS was: Re: UNIX of choice these days?
On Thu, Sep 28, 2017 at 10:09 AM, Theodore Ts'o <tytso(a)mit.edu> wrote:
One ops people had a sniff of AFS, they never wanted
to go
back to NFS.
Amen... and once you switched to a token manager or dlm under the covers
to do the locking, you never could understand why wouldn't do it properly.
Although the brilliance of NFS was its easy of getting out there getting
people addicted to using a distributed FS. That was what Sun nailed.
And just like communism had to add 'work incentives' -- NFS added state :-)
Clem
2424
days inactive
2424
days old
2 comments
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participants (3)
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Clem Cole -
jnc＠mercury.lcs.mit.edu -
Theodore Ts'o