> Von: John Stoffel [john@xxxxxxxxxxx]
> Gesendet: Sonntag, 24. August 2014 14:46
> An: Markus Stockhausen
> Cc: linux-raid@xxxxxxxxxxxxxxx
> Betreff: Re: [PATCH v3 5/6] md/raid5: activate raid6 rmw feature
> ...
> Also, how does the patch look when you have a simple 4 disk RAID6 array?
> I would hope that since the parity overhead is much higher, it would show
> more improvement as well.
Maybe I did not explain exactly what this patch is for. To boil it down to four
short and simple key facts.
- The current kernel implementation of RAID6 is very I/O expensive for small
changes. We always need to read all unchanged blocks of the same stripe,
recalulate the parity and write the new data block plus two parities. So a single
block change will produce n (number of disks) I/Os. The source code calls it
RCU - reconstruct write.
- With the patch we will only read the old data of the to be changed block & the
parities, recalculate the parities with the new data and rewrite everthing back. A
single block change will only have 6 I/Os regardless of the number of disks. It
is called RMW - read modify write.
- This patch will only kick in if you have 7 or more disks in a RAID6. And ONLY
if I/Os RMW < I/Os RCU. This should answer your question above.
- The more disks we have and the smaller the changed data is the more benefit
we will get from that patch.
Markus
****************************************************************************
Diese E-Mail enthält vertrauliche und/oder rechtlich geschützte
Informationen. Wenn Sie nicht der richtige Adressat sind oder diese E-Mail
irrtümlich erhalten haben, informieren Sie bitte sofort den Absender und
vernichten Sie diese Mail. Das unerlaubte Kopieren sowie die unbefugte
Weitergabe dieser Mail ist nicht gestattet.
�ber das Internet versandte E-Mails können unter fremden Namen erstellt oder
manipuliert werden. Deshalb ist diese als E-Mail verschickte Nachricht keine
rechtsverbindliche Willenserklärung.
Collogia
Unternehmensberatung AG
Ubierring 11
D-50678 Köln
Vorstand:
Kadir Akin
Dr. Michael Höhnerbach
Vorsitzender des Aufsichtsrates:
Hans Kristian Langva
Registergericht: Amtsgericht Köln
Registernummer: HRB 52 497
This e-mail may contain confidential and/or privileged information. If you
are not the intended recipient (or have received this e-mail in error)
please notify the sender immediately and destroy this e-mail. Any
unauthorized copying, disclosure or distribution of the material in this
e-mail is strictly forbidden.
e-mails sent over the internet may have been written under a wrong name or
been manipulated. That is why this message sent as an e-mail is not a
legally binding declaration of intention.
Collogia
Unternehmensberatung AG
Ubierring 11
D-50678 Köln
executive board:
Kadir Akin
Dr. Michael Höhnerbach
President of the supervisory board:
Hans Kristian Langva
Registry office: district court Cologne
Register number: HRB 52 497
****************************************************************************
