From: David Brown <david@westcontrol.com>
To: linux-raid@vger.kernel.org
Subject: Re: misunderstanding of spare and raid devices? - and one question more
Date: Fri, 01 Jul 2011 09:23:43 +0200 [thread overview]
Message-ID: <iujspj$jgg$1@dough.gmane.org> (raw)
In-Reply-To: <20110701072855.69ee763b@notabene.brown>
On 30/06/2011 23:28, NeilBrown wrote:
> On Thu, 30 Jun 2011 16:21:57 +0200 Karsten Römke<k.roemke@gmx.de> wrote:
>
>> Hi Phil
>>>
>>> If your CPU has free cycles, I suggest you run raid6 instead of raid5+spare.
>>>
>>> Phil
>>>
>> I started the raid 6 array and get:
>>
>> Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
>> md0 : active raid6 sde5[4] sdd5[3] sdc5[2] sdb2[1] sda3[0]
>> 13759296 blocks level 6, 64k chunk, algorithm 2 [5/5] [UUUUU]
>> [=================>...] resync = 87.4% (4013184/4586432) finish=0.4min speed=20180K/sec
> ^^^^^^
> Note: resync
>
>>
>> when I started the raid 5 array I get
>>
>> md0 : active raid5 sdd5[4] sde5[5](S) sdc5[2] sdb2[1] sda3[0]
>> 13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
>> [=>...................] recovery = 6.2% (286656/4586432) finish=0.9min speed=71664K/sec
> ^^^^^^^^
> Note: recovery.
>
>>
>> so I have to expect a three times less write speed - or is this calculation
>> to simple ?
>>
>
> You are comparing two different things, neither of which is write speed.
> If you want to measure write speed, you should try writing and measure that.
>
> When you create a RAID5 mdadm deliberately triggers recovery rather than
> resync as it is likely to be faster. This is why you see a missed device and
> an extra spare. I don't remember why it doesn't with RAID6.
>
What's the difference between a "resync" and a "recovery"? Is it that a
"resync" will read the whole stripe, check if it is valid, and if it is
not it then generates the parity, while a "recovery" will always
generate the parity?
If that's the case, then one reason it might not do that with raid6 is
if the code is common with the raid5 to raid6 grow case. Then a
"resync" would leave the raid5 parity untouched, so that the set keeps
some redundancy, whereas a "recovery" would temporarily leave the stripe
unprotected.
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next prev parent reply other threads:[~2011-07-01 7:23 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-30 10:51 misunderstanding of spare and raid devices? Karsten Römke
2011-06-30 10:58 ` Robin Hill
2011-06-30 13:09 ` Karsten Römke
2011-06-30 11:30 ` John Robinson
2011-06-30 12:32 ` Phil Turmel
2011-06-30 12:52 ` misunderstanding of spare and raid devices? - and one question more Karsten Römke
2011-06-30 13:34 ` Phil Turmel
2011-06-30 14:05 ` Karsten Römke
2011-06-30 14:21 ` Karsten Römke
2011-06-30 14:44 ` Phil Turmel
2011-07-02 8:34 ` Karsten Römke
2011-07-02 9:42 ` David Brown
2011-06-30 21:28 ` NeilBrown
2011-07-01 7:23 ` David Brown [this message]
2011-07-01 8:50 ` Robin Hill
2011-07-01 10:18 ` David Brown
2011-07-01 11:29 ` Robin Hill
2011-07-01 12:45 ` David Brown
2011-07-01 13:02 ` NeilBrown
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