Re: [PATCH 3/6] mke2fs: set block_validity as a default mount option

[Theodore Ts'o <tytso@mit.edu>]

On Fri, Aug 08, 2014 at 09:26:30PM -0700, Darrick J. Wong wrote:
> The block_validity mount option spot-checks block allocations against
> a bitmap of known group metadata blocks.  This helps us to prevent
> self-inflicted catastrophic failures such as trying to "share"
> critical metadata (think bitmaps) with file data, which usually
> results in filesystem destruction.
> 
> In order to test the overhead of the mount option, I re-used the speed
> tests in the metadata checksum testing script.  In short, the program
> creates what looks like 15 copies of a kernel source tree, except that
> it uses fallocate to strip out the overhead of writing the file data
> so that we can focus on metadata overhead.  On a 64G RAM disk, the
> overhead was generally about 0.9% and at most 1.6%.  On a 160G USB
> disk, the overhead was about 0.8% and peaked at 1.2%.

I was doing a spot check of the additional memory impact of
block_validity mount option, and it's for a 20T file system, assuming
the basic flex_bg size of 16 block groups, it's a bit over 400k of
kernel memory.  That's not a *huge* amount of memory, but it could
potentially be noticeable on a bookshelf NAS server.

However, I could imagine that for a system with say, two dozen 10T
drives (which aren't that far off in the future) in a tray, that's
around 4 megabytes of memory, which starts being non-trivial.

That being said, I suspect for most users, it's not that big of a deal
--- so maybe this is something we should just simply enable by default
in the kernel, let those folks who want to disable specify a
noblock_validity mount option.

The other thing to consider is that for big raid arrays, maybe we
should use a larger flex_bg size.  The main reason for keeping the
size small is to minimize the seek time between the inode table and a
block in the flex_bg.  But for raid devices, we could probably afford
to increase flex_bg size, which would decrease the numer of system
zones that the block validity code would need to track.

      	       	     	      	   	      - Ted