From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cloudserver020138.home.net.pl ([62.129.239.72]:44212 "EHLO cloudserver020138.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754476AbdFWRcI (ORCPT ); Fri, 23 Jun 2017 13:32:08 -0400 Message-ID: <594D4F03.5050507@sarach.com.pl> Date: Fri, 23 Jun 2017 19:25:23 +0200 From: =?UTF-8?B?TWljaGHFgiBTb2tvxYJvd3NraQ==?= MIME-Version: 1.0 To: linux-btrfs@vger.kernel.org Subject: Re: Exactly what is wrong with RAID5/6 References: <1f5a4702-d264-51c6-aadd-d2cf521a45eb@dirtcellar.net> In-Reply-To: <1f5a4702-d264-51c6-aadd-d2cf521a45eb@dirtcellar.net> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms010604030309010409010705" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms010604030309010409010705 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Hello group. I am confused: Can somebody please confirm/deny, which RAID subsystem is affected? BTRFS' RAID5/6 or mdadm (Linux kernel raid) RAID 5/6 ? Are there some gotchas (in terms of broken reliability) when using kernel one? The web is full of legends, it seems that this confusion is quite common.= =2E. On 06/21/2017 12:57 AM, waxhead wrote: > I am trying to piece together the actual status of the RAID5/6 bit of > BTRFS. > The wiki refer to kernel 3.19 which was released in February 2015 so I > assume that the information there is a tad outdated (the last update > on the wiki page was July 2016) > https://btrfs.wiki.kernel.org/index.php/RAID56 > > Now there are four problems listed > > 1. Parity may be inconsistent after a crash (the "write hole") > Is this still true, if yes - would not this apply for RAID1 / RAID10 > as well? How was it solved there , and why can't that be done for RAID5= /6 > > 2. Parity data is not checksummed > Why is this a problem? Does it have to do with the design of BTRFS > somehow? > Parity is after all just data, BTRFS does checksum data so what is the > reason this is a problem? > > 3. No support for discard? (possibly -- needs confirmation with cmason)= > Does this matter that much really?, is there an update on this? > > 4. The algorithm uses as many devices as are available: No support for > a fixed-width stripe. > What is the plan for this one? There was patches on the mailing list > by the SnapRAID author to support up to 6 parity devices. Will the > (re?) resign of btrfs raid5/6 support a scheme that allows for > multiple parity devices? > > I do have a few other questions as well... > > 5. BTRFS does still (kernel 4.9) not seem to use the device ID to > communicate with devices. > > If you on a multi device filesystem yank out a device, for example > /dev/sdg and it reappear as /dev/sdx for example btrfs will still > happily try to write to /dev/sdg even if btrfs fi sh /mnt shows the > correct device ID. What is the status for getting BTRFS to properly > understand that a device is missing? > > 6. RAID1 needs to be able to make two copies always. E.g. if you have > three disks you can loose one and it should still work. What about > RAID10 ? If you have for example 6 disk RAID10 array, loose one disk > and reboots (due to #5 above). Will RAID10 recognize that the array > now is a 5 disk array and stripe+mirror over 2 disks (or possibly 2.5 > disks?) instead of 3? In other words, will it work as long as it can > create a RAID10 profile that requires a minimum of four disks?=20 --------------ms010604030309010409010705 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIILDzCC BSUwggQNoAMCAQICEGaJgd+Elrs3JDTMTgSd0fQwDQYJKoZIhvcNAQELBQAwdTELMAkGA1UE BhMCSUwxFjAUBgNVBAoTDVN0YXJ0Q29tIEx0ZC4xKTAnBgNVBAsTIFN0YXJ0Q29tIENlcnRp ZmljYXRpb24gQXV0aG9yaXR5MSMwIQYDVQQDExpTdGFydENvbSBDbGFzcyAxIENsaWVudCBD QTAeFw0xNjA3MjkxMjA0MjRaFw0xOTEwMjkxMjA0MjRaMGwxJjAkBgNVBA0MHUZvciBsb2dp biBhdXRoZW50aWNhdGlvbiBvbmx5MR0wGwYDVQQDDBRtaWNoYWxAc2FyYWNoLmNvbS5wbDEj MCEGCSqGSIb3DQEJARYUbWljaGFsQHNhcmFjaC5jb20ucGwwggEiMA0GCSqGSIb3DQEBAQUA A4IBDwAwggEKAoIBAQDD6nNl0HXUSQbyTtcgn6++PUyDTzlkutl1Gao/bT1OMbapBroQoMNr 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