PERC/6i performance : RAID 0 vs RAID 10

[Priyank Patel]

Hi,
We are testing out the write performance of the PERC/6i RAID controller with
poweredge servers.  Here are some very surprising numbers from dd (with our
without O_DIRECT does not seem to matter) - using linux kernel - 2.6.20

---

scsi0 : LSI Logic SAS based MegaRAID driver
scsi 0:0:0:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:1:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:2:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:3:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:4:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:5:0: Direct-Access     HITACHI  HUS154545VLS300  A4C2 PQ: 0 ANSI: 5
scsi 0:0:32:0: Enclosure         DP       BACKPLANE        1.05 PQ: 0 ANSI: 5
scsi 0:2:0:0: Direct-Access     DELL     PERC 6/i         1.11 PQ: 0 ANSI: 5

---

dd results

RAID 10 :
Write ~ 276 MBps

RAID 0 :
Write ~ 25 MBps
Read ~ 540 MBps

Its pretty confusing why the write performance with a RAID 0 across 6 disks
is so dismal.  We have tried using O_DIRECT, thereby getting rid of
file-system caching effects...

Any idea whats going on over here ?

Thanks,
-pkpatel

PS : PERC/5i write performance seems to be as expected (we dont have the
bencharmark numbers for this yet, but there have been no issues so far)
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