After doing some measurements I can confirm that "write back" is actually working after all. Seems like this is opaque to the OS - which makes sense I suppose. Slightly curious why we get "Write Cache is Disabled" in the 32bit diagnostics though. These figures give quite a nice graph for those who are into that kind of thing (note the spike at data sent = 2 x bufsize) Data Sent(Mb) vs Transfer Rate(Mb/s) 601 347.398843930636 611 391.666666666667 621 363.157894736842 641 335.602094240838 661 277.731092436975 681 302.666666666667 701 273.828125 721 256.583629893238 741 242.156862745098 761 232.012195121951 781 224.425287356322 801 216.486486486486 901 191.295116772824 1001 175.922671353251 1101 173.11320754717 1201 170.354609929078 1401 171.901840490798 2001 196.176470588235
I replaced a failed 5iR with a reconditioned perc 5i from ebay. The system is a pe1900 (I) with  a single 70Gb dell SAS. On initial boot there were errors about the battery charge - I left the system to charge. On booting again I did ctrl-R  and initialized the disk as RAID0 with default settings + "write back" (this was not shown as default) . No errors or warnings this time. I did (is this right ?) to get out of the virtual disk manager and to reboot  Then I installed Fedora 17 from a CD with out any obvious problems. However on closer inspection the system boot log concerning the drive says " Assuming drive cache: write through" here's the rest :  megasas: 00.00.06.14-rc1 Fri. Jan. 6 17:00:00 PDT 2012 megasas: 0x1028:0x0015:0x1028:0x1f03: bus 2:slot 14:func 0 megaraid_sas 0000:02:0e.0: PCI IRQ 142 -> rerouted to legacy IRQ 18 megasas: FW now in Ready state megasas_init_mfi: fw_support_ieee=0 megasas: INIT adapter done scsi2 : LSI SAS based MegaRAID driver megaraid_sas: fw state:c0000000 megasas: fwstate:c0000000, dis_OCR=0 scsi 2:0:4:0: Direct-Access     MAXTOR   ATLAS10K5_073SAS BP00 PQ: 0 ANSI: 5 scsi 2:2:0:0: Direct-Access     DELL     PERC 5/i         1.00 PQ: 0 ANSI: 5 142082048 512-byte logical blocks: (72.7 GB/67.7 GiB) Test WP failed, assume Write Enabled Asking for cache data failed Assuming drive cache: write through sd 2:2:0:0: Attached scsi generic sg1 type 0 Test WP failed, assume Write Enabled Asking for cache data failed Assuming drive cache: write through  sda: sda1 sda2 Test WP failed, assume Write Enabled Asking for cache data failed Assuming drive cache: write through Attached SCSI disk For want of a better idea, I repeated this process and got the same result. Oh and hdparm gives the following: /dev/sda:SG_IO: bad/missing sense data, sb[]:  70 00 05 00 00 00 00 0b 00 00 00 00 20 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 multcount     =  0 (off) readonly      =  0 (off) readahead     = 256 (on) geometry      = 8844/255/63, sectors = 142082048, start = 0I don't really know what this means and google gives nothing I can understand about the SG_IO line. But I did leave the Read Policy at default "no read ahead" in the VDM. I guess next I will try the "restore factory defaults" option in the VDM. I am a software guy with a woeful hardware knowledge, can does any of this raise any flags for anyone ? Am I missing some step (quite possible) ? Are there any other diagnostics I can try ? And just to reiterate : I did initially get an amber light and a battery not charged warning - but have not seen anything since.
pe1900 perc 5i can't enable write back 