RE: [PATCH v4 1/3]mmc: set max_discard_sectors value for mmc queue

["Dong, Chuanxiao" <chuanxiao.dong@intel.com>]

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@arndb.de]
> Sent: Saturday, February 12, 2011 4:38 PM
> To: Dong, Chuanxiao
> Cc: linux-mmc@vger.kernel.org; cjb@laptop.org; linux-kernel@vger.kernel.org;
> akpm@linux-foundation.org; adrian.hunter@nokia.com
> Subject: Re: [PATCH v4 1/3]mmc: set max_discard_sectors value for mmc queue
> 
> On Saturday 12 February 2011 07:22:14 Chuanxiao Dong wrote:
> > max_discard_sectors value is UINT_MAX which means kernel block layer can pass
> > down unlimited sectors to MMC driver to erase. But erasing so many sectors may
> > delay some other important I/O requests. This is not preferred.
> >
> > So use 'pref_erase' to set a suitable max_discard_sectors value for mmc queue to
> > avoid erasing too many sectors at one time.
> >
> > Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
> 
> I'm not sure about this one. pref_erase on SDHC cards should be the *minimum*
> unit you can erase in one request, not the maximum. Erasing an arbitrary
Hi Arnd,
I found the comment in mmc_init_erase() function which explains the 'pref_erase':
'pref_erase' is defined as a guide to limit erases to that size and alignment.
So I think it is not the minimum unit driver can erase, also not the maximum erase unit. It just a guide erase size for driver which can avoid holding host controller too long to response other I/O requests.

> number of allocation units on an SDHC card should complete almost instantly,
> because it only needs to update a single table with the allocation units.
>
> Discarding partial allocation units will take a lot longer, because the
> card then has to copy over the remaining blocks.
Is discarding started when card is idle or right after erase command? Can it cause some other I/O requests delayed?