From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C812DC43219 for ; Wed, 12 Jan 2022 17:01:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355412AbiALRBq (ORCPT ); Wed, 12 Jan 2022 12:01:46 -0500 Received: from mout.web.de ([212.227.15.3]:46721 "EHLO mout.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349807AbiALRBp (ORCPT ); Wed, 12 Jan 2022 12:01:45 -0500 X-Greylist: delayed 313 seconds by postgrey-1.27 at vger.kernel.org; Wed, 12 Jan 2022 12:01:44 EST DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=web.de; s=dbaedf251592; t=1642006904; bh=VpwX7tXhTbrLuzi1YF8Uzmb4A2oBhdUT14fAMPWrFis=; h=X-UI-Sender-Class:Date:From:To:Cc:Subject:In-Reply-To:References; b=PeskKWovL/ACSiPTBh401vUzji0my+TACRDRioT7OKxnbJRg38vK9L8VFKIJbqPcI KXjyohg8wbSuXw6mRJ/2iQZJDVi6V7V1HeWGMM4MpinLd1V/Nl02zr+lUveXqI73dF dKp91ZyAl1wVDk5yZVzUixu1s0L2gub2JgC+352M= X-UI-Sender-Class: c548c8c5-30a9-4db5-a2e7-cb6cb037b8f9 Received: from gecko ([46.223.151.70]) by smtp.web.de (mrweb006 [213.165.67.108]) with ESMTPSA (Nemesis) id 1M91Pe-1n1fJI3tgc-006XPY; Wed, 12 Jan 2022 17:56:15 +0100 Date: Wed, 12 Jan 2022 16:56:12 +0000 From: Lukas Straub To: Qu Wenruo Cc: David Woodhouse , Linux FS Devel , "linux-block@vger.kernel.org" , "dm-devel@redhat.com" , linux-raid@vger.kernel.org Subject: Re: [dm-devel] Proper way to test RAID456? Message-ID: <20220112155351.5b670d81@gecko> In-Reply-To: <24998019-960c-0808-78df-72e0d08c904e@gmx.com> References: <0535d6c3-dec3-fb49-3707-709e8d26b538@gmx.com> <20220108195259.33e9bdf0@gecko> <20220108202922.6b00de19@gecko> <5ffc44f1-7e82-bc85-fbb1-a4f89711ae8f@gmx.com> <24998019-960c-0808-78df-72e0d08c904e@gmx.com> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="Sig_/trK2B+bQZg4U+J+J8zLUK/8"; protocol="application/pgp-signature"; micalg=pgp-sha256 X-Provags-ID: V03:K1:heywFokXSb224cpwvQrsi2v/LXbEHOE0r74vVZ0tAW61GPjbn6c eDdc9ppwND/oggCfxDn5DN/mNAEMOVaVO4sSRcD5N3Lu0QPeZ0mcwnxvixwiuMB9gADj3xX dVwYWnfBfnpweGh1YlYWJL76eTBrK6kgl6tA9RTyCdIlzfqKoVwEtzccIk2kpvTVW8uMHiR nbXSeKAdNKk+mM7NioWjQ== X-UI-Out-Filterresults: notjunk:1;V03:K0:HoFGdC2jWiw=:IO49La0OrTdQ2Ah/QSu86R PInkEFYz0f7sLMKmILByyxEbjUOcCQgktOoaHafjY777p5GLH7bbCdB8Jz7611+wkdnpBRu7r uOsURFWjr4Uv/Va3/HJqSR2CzbbVn/T4mQUwGUfZe+gJF9uOHT1FEj2hL8swfEpA2hKBagFFG ahOztmFrt5u5vJ+7cSOi66FDfXwg9xr7L+ocsKIuXF/gA0XBzD4/9jsCrEtjlIhgfw4L/JcVT E6XNq63820FwJ/iQvuDpBIH7/1f5WVZBonMyexMx7FEDeE98Q1hfz+DbMiFXMUDMajCB3yAgX aRhDNXsrCu0seOZLJYeQNzhjJ7q3MmVx/71Ur8SzphSEKAm/Yp1bCE4Nqcyx19SZxUSSgJfso MeWdPL9FHR+oRO8iNFcc6YA9p52dilbL6s7ycJbAr7RwVpXs/EZaD/3CZMIuKKFcwtkb8w+gA DfKTiGgkSf7PndQWkSk260pTzE9gj6L9HpgoS74v5hXd27wuXTQxjM6UiAjcl0ujB3+TUni3e cTO0EOcBYgpy85HayzEQMcnWDbP7A+Wia9gkjREnmEwhjLeBkWENf83z1uOo2AiX7V+pwgwv1 qMOLIOomVl+0Tr+FpBW+X4pFXqkvS+TEpeRgKny27cY00YaWBhLoiNXnIKAEidd/qA5WQbIJ5 alM8JfrfHK4cmMBtX+E/zwqH/HYaOtP58bwh8pjyNfJOjgqe0ym3ej6CcfCMHP3ci1YhzhmbL sIMjcCUlLCD5ILBna9NQElbEAfcSACWI/bkz+g4kAehupxO6RM+2C6Msg7t0l/uclPPw6+wS+ d7OHze5YbfgRLNDHLyxYbl3cwM6zLgWj0i4pdXXSiinBcnh+JNndgXR4TR/Mcg6BqEcQJDJ+U O3x24kK3e9PlhPSxSuHspAc1qoSqghi86xAX8NCS2y95PAceQbjOv01dZDBQCijS/L3xo8aTt sdhGSC2NrB4ibyEf6PEKhxNZINhJX4JlFi9/VgOcnhsbyNjXw99oAo5wq99zxF50iolzY2bzw U4/2Xn2bc4Sb9HdUEj8iOqHOx0g8+IWNXuvX7S3ZnfQbINbYU8F07rFS+GO+YZOtVaEBSZg1W 4GFIFl2Xg7/rdQ= Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org --Sig_/trK2B+bQZg4U+J+J8zLUK/8 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Sun, 9 Jan 2022 20:13:36 +0800 Qu Wenruo wrote: > On 2022/1/9 18:04, David Woodhouse wrote: > > On Sun, 2022-01-09 at 07:55 +0800, Qu Wenruo wrote: =20 > >> On 2022/1/9 04:29, Lukas Straub wrote: =20 > >>> But there is a even simpler solution for btrfs: It could just not tou= ch > >>> stripes that already contain data. =20 > >> > >> That would waste a lot of space, if the fs is fragemented. > >> > >> Or we have to write into data stripes when free space is low. > >> > >> That's why I'm trying to implement a PPL-like journal for btrfs RAID56= . =20 > > > > PPL writes the P/Q of the unmodified chunks from the stripe, doesn't > > it? =20 >=20 > Did I miss something or the PPL isn't what I thought? >=20 > I thought PPL either: >=20 > a) Just write a metadata entry into the journal to indicate a full > stripe (along with its location) is going to be written. >=20 > b) Write a metadata entry into the journal about a non-full stripe > write, then write the new data and new P/Q into the journal >=20 > And this is before we start any data/P/Q write. >=20 > And after related data/P/Q write is finished, remove corresponding > metadata and data entry from the journal. >=20 > Or PPL have even better solution? Yes, PPL is a bit better than a journal as you described it (md supports both). Because a journal would need to be replicated to multiple devices (raid1) in the array while the PPL is only written to the drive containing the parity for the particular stripe. And since the parity is distributed across all drives, the PPL overhead is also distributed across all drives. However, PPL only works for raid5 as you'll see. PPL works like this: Before any data/parity write either: a) Just write a metadata entry into the PPL on the parity drive to indicate a full stripe (along with its location) is going to be written. b) Write a metadata entry into the PPL on the parity drive about a non-full stripe write, including which data chunks are going to be modified, then write the XOR of chunks not modified by this write in to the PPL. To recover a inconsistent array with a lost drive: In case a), the stripe consists only of newly written data, so it will be affected by the write-hole (this is the trade-off that PPL makes) so just standard parity recovery. In case b), XOR what we wrote to the PPL (the XOR of chunks not modified) with the modified data chunks to get our new (consistent) parity. Then do standard parity recovery. This just works if we lost a unmodified data chunk. If we lost a modified data chunk this is not possible and just do standard parity recovery from the beginning. Again, the newly written data is affected by the write-hole but existing data is not. If we lost the parity drive (containing the PPL) there is no need to recover since all the data chunks are present. Of course, this was a simplified explanation, see drivers/md/raid5-ppl.c for details (it has good comments with examples). This also covers the case where a data chunk is only partially modified and the unmodified part of the chunk also needs to be protected (by working on a per-block basis instead of per-chunk). The PPL is not possible for raid6 AFAIK, because there it could happen that you loose both a modified data chunk and a unmodified data chunk. Regards, Lukas Straub > > > > An alternative in a true file system which can do its own block > > allocation is to just calculate the P/Q of the final stripe after it's > > been modified, and write those (and) the updated data out to newly- > > allocated blocks instead of overwriting the original. =20 >=20 > This is what Johannes is considering, but for a different purpose. > Johannes' idea is to support zoned device. As the physical location a > zoned append write will only be known after it's written. >=20 > So his idea is to maintain another mapping tree for zoned write, so that > full stripe update will also happen in that tree. >=20 > But that idea is still in the future, on the other hand I still prefer > some tried-and-true method, as I'm 100% sure there will be new > difficulties waiting us for the new mapping tree method. >=20 > Thanks, > Qu >=20 > > > > Then the final step is to free the original data blocks and P/Q. > > > > This means that your RAID stripes no longer have a fixed topology; you > > need metadata to be able to *find* the component data and P/Q chunks... > > it ends up being non-trivial, but it has attractive properties if we > > can work it out. =20 --=20 --Sig_/trK2B+bQZg4U+J+J8zLUK/8 Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEg/qxWKDZuPtyYo+kNasLKJxdslgFAmHfCCwACgkQNasLKJxd slj5YRAAriTi+zPKCzdoJ0d2cj8ZkHXtVoQMa5qr3R5f20aEyu/oTPkMFvmtsOKj ViMcFhTaV5Ceardq7Td7G3N2QzBJjEQsrg0FIl6aD9Jp8XWW49VwXRTyHihXi7xy HRCziWCxp8HWc6qy3IpOCm/XD3i0i37IDrm8dmA7r62swOALFX5g5zIpDo/CkfNU Tln3wA97KMEd93dWLzvWZsXFhETQ2lX9l5EMYzTjUqR0PAtbVJlHfeVYTnF1eAnw S5+TyiwfXa0lqOKUABYR8Ygnelu98VROpNWXUJi25CaY8l2T5jigskFdNvm+Ihy+ era4R8dAsXCBhTEmO4JXNLmE42IU+m+6DnZlGHxn+v3wuh7Tz46sAAC9JX73R6Fk e925eXQ2e2df7Ej964r3f4u/yiVO+/MaoiLUacz+U3VbZu9LOGYJ+xmnmycjhV3X NmfBV1nnWIeYU6fHvRigLCMJXnVd9dPlYlqW0alRq/OtLW2rKlUkXf+RWLOpT8By +ZMyloNz0/84MFLN+XhgdbwpEJ/NStbaJXJtD7T2nUf2l3+kWV6rFrmPIgxdPYyb 7yJMWxj9rGaU+2XmAkqoyC5ahj30apruZbQLyCXqc1EeHlGY8/Wrq5h2ZBAJcDbr CH22cVV2JzDT77MvzC+9nPWT/rkm8MNna4TRLZgtTauio9bPGHg= =Jly6 -----END PGP SIGNATURE----- --Sig_/trK2B+bQZg4U+J+J8zLUK/8--