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fix make-bcache to use logical_block_size, not stat.st_blocksize

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As discussed on irc yesterday.

Cheers,
	Lars

From 791a4b7b2c25e21ffeb4184da5e61f18cde86246 Mon Sep 17 00:00:00 2001
From: Lars Ellenberg <lars@linbit.com>
Date: Wed, 17 Jul 2013 11:49:12 +0200
Subject: [PATCH] make-bcache: fix guessing of "block_size" to use BLKSSZGET

Using stat.st_blocksize is wrong:
that is the linux buffer cache granularity,
it is tunable, and defaults to 4k.

This caused bcache to default to create devices with
logical_block_size = 4k, potentially breaking otherwise healthy disk
or file system images unnecessarily.

e.g. some xfs image previously created with 512 byte "sector size"
would, once bcache'd, refuse to mount with
	mount: function not implemented
	device supports 4096 byte sectors (not 512)

We want the logical_block_size here.
This commit is contained in:
Lars Ellenberg 2013-07-17 12:39:14 +02:00 committed by Kent Overstreet
parent 557c79518c
commit 7721b11a60
1 changed files with 31 additions and 0 deletions
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31
make-bcache.c
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@ -269,6 +269,37 @@ static unsigned get_blocksize(const char *path)
exit(EXIT_FAILURE);
}
if (S_ISBLK(statbuf.st_mode)) {
/* check IO limits:
* BLKALIGNOFF: alignment_offset
* BLKPBSZGET: physical_block_size
* BLKSSZGET: logical_block_size
* BLKIOMIN: minimum_io_size
* BLKIOOPT: optimal_io_size
*
* It may be tempting to use physical_block_size,
* or even minimum_io_size.
* But to be as transparent as possible,
* we want to use logical_block_size.
*/
unsigned int logical_block_size;
int fd = open(path, O_RDONLY);
if (fd < 0) {
fprintf(stderr, "open(%s) failed: %m\n", path);
exit(EXIT_FAILURE);
}
if (ioctl(fd, BLKSSZGET, &logical_block_size)) {
fprintf(stderr, "ioctl(%s, BLKSSZGET) failed: %m\n", path);
exit(EXIT_FAILURE);
}
close(fd);
return logical_block_size / 512;
}
/* else: not a block device.
* Why would we even want to write a bcache super block there? */
return statbuf.st_blksize / 512;
}