linux-kernel - Re: [PATCH] brd: detect zero writes for saving ram

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Date:	Sun, 8 Feb 2015 13:03:34 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Akinobu Mita <akinobu.mita@...il.com>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] brd: detect zero writes for saving ram

2015-02-08 0:18 GMT+09:00 Akinobu Mita <akinobu.mita@...il.com>:
> 2015-02-08 0:03 GMT+09:00 Akinobu Mita <akinobu.mita@...il.com>:
>> This introduces a module parameter to detect zero writes and not to
>> allocate memory.  Read requests for unallocated (unwritten) region
>> end up by reading zero.  So this can save zeroed memory consumption
>> with extra overhead for the detection.
>>
>> This feature is useful for testing filesystems and user programs to
>> huge files without huge real storage.  So this change also extends
>> the upper limit on the size of the RAM disk.
>
> I have just noticed about zram and I'll check I can use it instead.

zram currently needs to allocate memory proportional to the size of
the disk ((disksize >> PAGE_SHIFT) * sizeof(struct zram_table_entry)).
If disksize is TB or PB class, it requires huge amount of memory.
But brd with this patch can create 1PB filesystem.  So there is a
reason for this new brd feature to exist.

        # modprobe brd zero_detect=1 rd_size=$((1024*1024*1024*1024))
        # mkfs.xfs /dev/ram0
        # mount /dev/ram0 /mnt
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