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Date: Mon, 16 Jan 2012 00:38:29 +0100
Message-ID: <CADf-Jb=2pbL=yveQwqVb0x9NVyfX0Z5jxdeM4-k3b1WSjuJ...@mail.gmail.com>
Subject: Re: happstack-state / acid-state and disk full
From: Lemmih <lem...@gmail.com>
To: happs@googlegroups.com
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On Mon, Jan 16, 2012 at 12:29 AM, Jeremy Shaw <jer...@n-heptane.com> wrote:
> On Sun, Jan 15, 2012 at 3:55 PM, Petter Bergman
> <jon.petter.berg...@gmail.com> wrote:
>
>>
>> What will happen if happstack-state can't write an event to disk because
>> the disk is full?
>
>
> Who knows. One of the reasons for a clean rewrite as acid-state is that
> happstack-state had too many loose ends like that.
>
>>
>> What will happen in the same situation with acid-state?
>
>
> One of the things Lemmih worked on in acid-state was ensuring that events
> really got flushed to disk. That is the reason why it uses the low-level
> posix API for writing files to disk.
>
> The correct behavior when the disk is full is for update transactions to
> fail. That is pretty much the only way to handle the 'durability' aspect =
of
> ACID. If we can not successfully flush the event log to disk, then we can
> not allow the transaction to complete.
>
> Internally, acid-state uses 'write(2)' to write the transaction to disk (=
or
> something similar on Win32). If the write fails, it will it raise an ERRN=
O
> exception ENOSPC.
>
> Still, it is not entirely obvious what the result of that will be. Lookin=
g
> at the code, =C2=A0I think it means that the 'update' call will never ret=
urn
> because the thread that writes the log will die and never write the mvar
> that allows update to return. Obviously.. it might be nicer if the update
> function itself raised an exception instead of just blocking? Though
> blocking forever (or until happstack kills the request thread) does seem
> ACID compliant..
>
> Hopefully Lemmih can provide a better answer.

It'll probably fail in some ugly way but it'll definitely be ACID
compliant. Making it fail more gracefully isn't high on my TODO list
since it isn't a torn in my side.

--=20
Cheers,
=C2=A0 Lemmih