zero deleted blocks

[Petri Helenius]

Obviously the most elegant way would be for KVM, Xen and VMware to support TRIM/UNMAP...

Pete

On Aug 13, 2012, at 11:58 , "Steven Hartland" <killing at multiplay.co.uk> wrote:

> ----- Original Message ----- From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
> 
> 
>>> I think TRIM support in UFS can be extended to zero-out the deleted
>>> blocks, with relatively small amount of work. Basically, in
>>> ufs/ffs/ffs_alloc.c:ffs_blkfree(), a BIO_WRITE of zero buffer shall be
>>> issued instead of BIO_DELETE.
>> It would be a better idea to give geom_disk a per-disk option to
>> convert BIO_DELETE to writes of zeros.
> 
> cam da already has this option with the sysctl:-
> kern.cam.da.X.delete_method = ZERO
> 
> This however requires the underlying SCSI device supports
> Write Same (WS10 or WS16) which seems to fairly rare.
> 
> I'm not aware of any an equivalent ATA command, but it would
> be possible to have both use a standard write to achieve that.
> 
> If this was done care should be take with regards performance
> as this will be significantly slower than the current methods.
> 
> That said if the underlying device "supports" trim / unmap
> it could interpret that how it sees fit.
> 
>   Regards
>   Steve
> 
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