From: Andreas Dilger Subject: Re: [PATCH] libext2fs: readahead for meta_bg Date: Tue, 28 Feb 2017 19:50:48 -0700 Message-ID: <4F4ECDB0-7939-4F3D-8995-0BA6A96C658E@dilger.ca> References: <1487585025-16654-1-git-send-email-artem.blagodarenko@gmail.com> <42AA3FB8-88C4-4616-A20F-D09F0833D288@dilger.ca> <62970C8A-AEB5-4AE8-8C83-C9BA41D313AB@gmail.com> Mime-Version: 1.0 (Mac OS X Mail 10.2 \(3259\)) Content-Type: multipart/signed; boundary="Apple-Mail=_0CC789C4-00E4-4E76-93D4-C7CA9EEE5834"; protocol="application/pgp-signature"; micalg=pgp-sha1 Cc: Artem Blagodarenko , linux-ext4 To: Alexey Lyashkov Return-path: Received: from mail-it0-f67.google.com ([209.85.214.67]:34451 "EHLO mail-it0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750709AbdCAFzq (ORCPT ); Wed, 1 Mar 2017 00:55:46 -0500 Received: by mail-it0-f67.google.com with SMTP id r141so3925916ita.1 for ; Tue, 28 Feb 2017 21:54:06 -0800 (PST) In-Reply-To: <62970C8A-AEB5-4AE8-8C83-C9BA41D313AB@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: --Apple-Mail=_0CC789C4-00E4-4E76-93D4-C7CA9EEE5834 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 On Feb 28, 2017, at 7:19 PM, Alexey Lyashkov = wrote: >=20 > Andreas, >=20 > we have do it first. But patch was much and much complex due checksums = handling in code. > and ext2_flush() will be need to read an all GD in memory to as use a = flat array to write a GD to the disk. Yes, I saw ext2_flush() was accessing the array directly, and would have = a problem as you describe. One option would be to skip writing = uninitialized GDT blocks, but that also becomes more complex to get correct. > If you want we have submit it also, but it have no benefits (like a = few seconds), against it simple version. I guess a large part of your speedup is because of submitting the GDT = reads in parallel to a disk array. If the GDT blocks are all mapped to a = single disk in the array (entirely possible with META_BG, depending on array = geometry) then the prefetch will have minimal benefits. Another option would be to change debugfs/tune2fs/dumpe2fs to use the EXT2_FLAG_SUPER_ONLY flag to only read the superblock on open if the requested operations do not need access to the group descriptors at all? For a large filesystem as you describe, 37K GDT blocks is still over = 144MB of data that needs to be read from disk, vs 4KB for the superblock. Cheers, Andreas >> 1 =D0=BC=D0=B0=D1=80=D1=82=D0=B0 2017 =D0=B3., =D0=B2 3:10, Andreas = Dilger =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB(=D0=B0= ): >>=20 >> On Feb 20, 2017, at 3:03 AM, Artem Blagodarenko = wrote: >>>=20 >>> From: Alexey Lyashkov >>>=20 >>> There are ~37k of random IOs with meta_bg option on 300T target. >>> Debugfs requires 20 minutes to be started. Enabling readahead for >>> group blocks metadata save time dramatically. Only 12s to start. >>>=20 >>> Signed-off-by: Alexey Lyashkov >>=20 >> This patch looks good by itself. >>=20 >> Reviewed-by: Andreas Dilger >> ---- >>=20 >> On a related note, I've been wondering if it would make sense to have >> a second patch that *only* does the readahead of the group descriptor = blocks >> in ext2fs_open2(), and move io_channel_read_blk64() to = ext2fs_group_desc() >> when the group descriptor blocks are actually accessed the first = time? This >> would allow tools like tune2fs, debugfs, dumpe2fs, etc. that may not = access >> group descriptors to load _much_ faster than if it loads all of the = bitmaps >> synchronously at filesystem open time. Even if they _do_ access the = GDT it >> will at least allow the prefetch more time to run in the background, = and the >> GDT swabbing happen incrementally upon access rather than all at the = start. >>=20 >> A quick look through lib/ext2fs looks like ext2fs_group_desc() is = used for >> the majority of group descriptor accesses, but there are a few places = that >> access fs->group_desc directly. The ext2fs_group_desc() code could = check >> whether the group descriptor is all-zero (ext2fs_open2() should be = changed >> to use ext2fs_get_array_zero(..., &fs->group_desc)) and if so read = the whole >> descriptor block into the array and optionally swab it. >>=20 >> Cheers, Andreas >>=20 >>> --- >>> lib/ext2fs/openfs.c | 6 ++++++ >>> 1 files changed, 6 insertions(+), 0 deletions(-) >>>=20 >>> diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c >>> index ba501e6..f158b0a 100644 >>> --- a/lib/ext2fs/openfs.c >>> +++ b/lib/ext2fs/openfs.c >>> @@ -399,6 +399,12 @@ errcode_t ext2fs_open2(const char *name, const = char *io_options, >>> #endif >>> dest +=3D fs->blocksize*first_meta_bg; >>> } >>> + >>> + for (i =3D first_meta_bg ; i < fs->desc_blocks; i++) { >>> + blk =3D ext2fs_descriptor_block_loc2(fs, group_block, = i); >>> + io_channel_cache_readahead(fs->io, blk, 1); >>> + } >>> + >>> for (i=3Dfirst_meta_bg ; i < fs->desc_blocks; i++) { >>> blk =3D ext2fs_descriptor_block_loc2(fs, group_block, = i); >>> retval =3D io_channel_read_blk64(fs->io, blk, 1, dest); >>> -- >>> 1.7.1 >>>=20 >>=20 >>=20 >> Cheers, Andreas >>=20 >>=20 >>=20 >>=20 >>=20 >=20 Cheers, Andreas --Apple-Mail=_0CC789C4-00E4-4E76-93D4-C7CA9EEE5834 Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP -----BEGIN PGP SIGNATURE----- Comment: GPGTools - http://gpgtools.org iD8DBQFYtjcIpIg59Q01vtYRAli9AJ4t44n0N2MpHrh5byR0kWwc8IP+cACg44SR IAK+ncPFeDr0zuEJ07CHl98= =ex2A -----END PGP SIGNATURE----- --Apple-Mail=_0CC789C4-00E4-4E76-93D4-C7CA9EEE5834--