Re: [PATCH]: add an option to disable direct I/O in shred

[Pádraig Brady]

On 15/10/15 12:46, Andreas Kinzler wrote:
> On 14.10.2015 18:48, Pádraig Brady wrote:
>>> some disks can be very slow with direct I/O and shred. I developed a
>>> small patch that adds a new
>>> option -i, --nodirectio to disable direct I/O.
>> Interesting. direct I/O was added in 2004 to improve performance
>> by avoid page cache thrashing, as suggested in:
>> https://bugs.debian.org/207035
>> Can you give us some details on where direct I/O is slower?
>> Are you working with files or device nodes?
>> Which type of disk device is this?
> 
> I use shred for block devices only. My measurements show that
> direct I/O is indeed optimal for block devices where the device has
> its write cache enabled. If for some reason the write cache is
> disabled and/or cannot be enabled, then shred with direct I/O is
> very slow because the device must complete a physical transaction
> for each 64 KB write request. In this case, the option to disable
> direct I/O is very helpful.

Thanks for the extra info.

> In my case the cache could not be enabled on some older SATA (via AHCI)
> drives.

That is an edge case in fairness.
I'd like to avoid an option for this performance only edge case.

Attached is a patch to periodically request to discard written pages,
rather than use O_DIRECT. I need to do a bit more testing to
ensure it introduces no performance regressions, though it's
worth noting that we ran without direct I/O between 2006-2013
without any requests to address that.

thanks,
Pádraig.

shred-avoid-direct-io.patch
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