linux-kernel - Re: [PATCH 3/3] tcm/fileio: Add UNMAP / Block DISCARD support

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Date:	Wed, 29 Sep 2010 10:57:56 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Douglas Gilbert <dgilbert@...erlog.com>,
	Jens Axboe <axboe@...nel.dk>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Hannes Reinecke <hare@...e.de>,
	James Bottomley <James.Bottomley@...e.de>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Richard Sharpe <realrichardsharpe@...il.com>
Subject: Re: [PATCH 3/3] tcm/fileio: Add UNMAP / Block DISCARD support

On 09/29/2010 10:08 AM, Nicholas A. Bellinger wrote:
> On Tue, 2010-09-28 at 08:46 +0200, Boaz Harrosh wrote:
>>> +			printk(KERN_WARNING "Ignoring UNMAP for non BD"
>>> +					" backend for struct file\n");
>>
>> TODO: Lots of extent based filesystems could enjoy if you punch an hole
>>       in the file. (Which will also send a proper discard).
>>       So how to punch an hole in a file via vfs?
> 
> Good question, as all of the current block discard support assumes a
> struct block_device * pointer currently.
> 
> I know that hch has mentioned that accesing a struct block_device from
> struct file is very dangerous, but I am assuming for the TCM/FILEIO case
> that this code will be protected by igrab() and iput().
> 
> hch and jaxboe, any comments here..?
> 

No you did not understand. FILEIO (working on a filesystem's real file),
should *never* attempt a discard on the blocks even if it was
to read the file's map and figure out these blocks. It should always
call a filesystem specific API that puntches-an-hole in the file.
Fore stupid filesystem that means fill with zero's. For a smart
extent-based file system it means splitting up the extents the range
belongs to and freeing up the block that where allocated for that
range. But it is an heavy meta-data operation that only the filesystem
can do. With really smart FS like xfs the unallocated blocks also
get discarded, for when working with SANs with over provisioning.

> Thanks!
> 
> --nab

Boaz
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