Re: [PATCH] libata: Whitelist SSDs that are known to properly return zeroes after TRIM

[Phillip Susi <psusi@ubuntu.com>]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 1/7/2015 11:58 PM, Andreas Dilger wrote:
>> No, it knows that the inode table needs initialized because there
>> is a flag in the group descriptor that says this inode table is
>> still uninitalized.  It never reads the blocks to see if they are
>> full of zeros.  mke2fs sets the flag when it does not initialize
>> the table with zeros, either by direct writes ( which it doesn't
>> do if lazy_itable_init is true, which it defaults to these days
>> ), or by discarding the blocks when the device claims to support
>> deterministic discard that zeros.
> 
> That is only partially correct.  While it is true that mke2fs sets
> the UNINIT flag at format time, the "lazy" part of that means there
> is a kernel thread still does the zeroing of the inode table
> blocks, but after the filesystem is mounted, for any group that
> does not have the ZEROED flag set.  After that point, the "UNINIT"
> flag is an optimization to avoid reading the bitmap and unused
> blocks from disk during allocation.

That is pretty much what I said, except that I was pointing out that
it does not *read* first to see if the disk is already zeroed, as that
would be a waste of time.  It just writes out the zeros for block
groups that still have the uninit flag set.

> This is needed in case the group descriptor or inode bitmap is
> corrupted, and e2fsck needs to scan the inode table for in-use
> inodes.  We don't want it to find old inodes from before the
> filesystem was formatted.
> 
> The ext4_init_inode_table() calls
> sb_issue_zeroout->blkdev_issue_zeroout(), so if the underlying
> storage supported deterministic zeroing of the underlying storage,
> this could be handled very efficiently.

Again, that's pretty much what I said.


-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.17 (MingW32)

iQEcBAEBAgAGBQJUro+rAAoJENRVrw2cjl5RDcMIAIF/qOjB4FfQ1v1NPLnbYrwa
MeGxmAGIq8ftOlfOpSo8W/Adal4eegPwiBzE/8JciTHli2hZFkboG96lCGLL2GHd
51gqADIcSUmPHleLP3L/MSleFLvOAC4xYWYi00idZF5UVK3tt6m07+T/DCkoku1L
8Yg0BH5rxxRyPMRe3a/Y3WoI3PFFQDiDhz/7PSFH1W6JrEmiZutIDfm/U4h6zFWU
tlNUMyiya8DrJt4katjBTa74EPWdZQjT/dOuUGGslAgOro69ZZQ4jmEXeJGN96Ly
mMhbzkw9mjvSHYWuH2Hf1QlxoMKgq1hxaNbwEtImBzYeQTb9VSqM6BSO1ru0m9M=
=X07B
-----END PGP SIGNATURE-----