linux-ext4 - Re: [PATCH 0/3] Batched discard support

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Date:	Fri, 06 Aug 2010 18:24:08 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-ext4@...r.kernel.org, jmoyer@...hat.com, rwheeler@...hat.com,
	eshishki@...hat.com, sandeen@...hat.com, jack@...e.cz,
	tytso@....edu
Subject: Re: [PATCH 0/3] Batched discard support

Lukas Czerner <lczerner@...hat.com> writes:

> On Fri, 6 Aug 2010, Dmitry Monakhov wrote:
>
>> Lukas Czerner <lczerner@...hat.com> writes:
>> 
>> > Hi, all
>> >
>> > because people were worried about possibly long stalls appearing
>> > when FITRIM ioctl is working, I have changed the FITRIM interface
>> > as Dimitry suggested. Now you can choose whether to trim whole
>> > file system or just a part of it, resp. you can specify the range
>> > of Bytes to trim.
>> Agree with whole patch-set, except minor note for ext4'th path.
>> Please feel free to add
>> Reviewed-by: Dmitry Monakhov <dmonakhov@...nvz.org> to the series
>> 
>> The only thing what is still not obvious for me is that, there are
>> several types of discard request possible
>> 1) Simple discard 
>> 2) Secure discard which was proposed here http://lkml.org/lkml/2010/6/24/71
>> Should we specify which type should be used in ioctl flags?
>> But i hope that we can just stick maximum security scenario
>> Use secure discard if possible.
>
> First of all, thanks for you review Dimitry. And second, to be honest I
> am not entirely familiar with the Secure discard implementation. Right
> now it just doing the simple discard like "send TRIM command", so it
> does work just for devices which supports it. I suppose we can just
> check blk_queue_discard() at some level and then decide whether to do
> simple discard (TRIM), or secure discard "Write zeroes", when the device
> does not support TRIM - if it is what you mean by secure discard.
I'm also not quite familiar with SECDISCARD feature also :)
BUT as soon as i understand MMC cards has ability to discard
with zeroes, and destroy backup blocks in one discard request.
At the same time secure discard emulation with writing zeroes will
take too long (hours and days) so i don't see any reason for emulation.
We have SECRM flag for this purpose.

>
> Regards
> -Lukas
>
>> >
>> > To be specific you can create something like this:
>> >
>> > int main(int argc, char **argv)
>> > {
>> > 	int fd;
>> > 	uint64_t range[3];
>> >
>> > 	range[0] = 40960;
>> > 	range[1] = 134217728;
>> > 	range[2] = 4096;
>> >
>> > 	fd = open(argv[1], O_RDONLY);
>> > 	if (fd < 0) {
>> > 		perror("open");
>> > 		return 1;
>> > 	}
>> >
>> > 	if (ioctl(fd, FITRIM, range)) {
>> > 		if (errno == EOPNOTSUPP)
>> > 			fprintf(stderr, "FITRIM not supported\n");
>> > 		else
>> > 			perror("FITRIM");
>> > 		return 1;
>> > 	}
>> >
>> > 	return 0;
>> > }
>> >
>> > Range items have following meaning:
>> >
>> > range[0] - (start) first Byte to trim
>> > range[1] - (len) number of Bytes to trim from start
>> > range[2] - (minlen) minimum extent length to trim, free extents shorter
>> > than this number of Bytes will be ignored. This number will be rounded
>> > up to the block size.
>> >
>> > So in my example it will trim all free extents from block 10 of first
>> > alloc. group to block 10 of second alloc. group, assuming we have
>> > block_size = 4096.
>> >
>> > Also, when you want to trim the whole fs, you can simply pass NULL
>> > instead of range into the ioctl, or you can specify the range correctly
>> > to cover the whole fs.
>> >
>> > Regards
>> > -Lukas
>> >
>> > [PATCH 1/3] Add ioctl FITRIM.
>> > [PATCH 2/3] Add batched discard support for ext3
>> > [PATCH 3/3] Add batched discard support for ext4
>> >
>> >  fs/ext3/balloc.c        |  249 +++++++++++++++++++++++++++++++++++++++++++++++
>> >  fs/ext3/super.c         |    1 +
>> >  fs/ext4/ext4.h          |    2 +
>> >  fs/ext4/mballoc.c       |  194 ++++++++++++++++++++++++++++++++++++
>> >  fs/ext4/super.c         |    1 +
>> >  fs/ioctl.c              |   34 +++++++
>> >  include/linux/ext3_fs.h |    1 +
>> >  include/linux/fs.h      |    2 +
>> >  8 files changed, 484 insertions(+), 0 deletions(-)
>> 
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