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CentOS mailing list <centos@xxxxxxxxxx> writes: Important: The partition you boot from must not be striped. It may not be raid-5 or raid-0. Note: On the one hand, if you want extra stability, consider using raid-1 (or even raid-5) for your swap partition(s) so that a drive failure would not corrupt your swap space and crash applications that are using it. On the other hand, if you want extra performance, just let the kernel use distinct swap partitions as it does striping by default. [quote] I have a number of older 3-bay SCSI systems that aren't worth adding a RAID card to, so the formula that gives me the best *OVERALL* performance for general-purpose use is: /dev/hda: hda1 - 100Mb - type FD hda2 - 512Mb - type FD hda3 - (rest of drive) - type FD /dev/hdb, /dev/hdc - identical setup to /dev/hda /dev/md0: RAID-1, 3 members, ext3fs, /boot /dev/md1: RAID-5, 3 members, swap /dev/md2: RAID-5, 3 members, ext3fs, / GRUB and LILO can both boot off RAID-1 sets. I don't need EVMS or LVM support, so I avoid that overhead. I don't need DM support either, so I avoid that overhead. The MD code automatically replicates RAID-1 data to every member equally, and balances read activity among all available members. The read performance boost is largely irrelevant since it's only the /boot filesystem. The RAID-5 implementation in the MD code is often faster than dedicated RAID controllers - these are dual-P-III 1.1GHz systems, and the MD5 RAID5 code outperforms the IBM ServeRAID 4M controller by about 50%. Of course I don't have a true hardware RAID controller with battery backup etc, either... Gains: -no RAID controller purchase -triple-redundant boot (requires GRUB to be installed on all 3 drives, see link posted previously) -swap on RAID, even swapped processes can survive a single-drive failure -root on RAID, system can survive a single-drive failure -very good read/write performance as long as CPU and I/O aren't maxed out simultaneously (which is rare for *my* workloads) -all drives have identical partition tables; recovery is simplified because initialization of a new drive consists of: 1. sfdisk -d /dev/sda | sfdisk /dev/sdb (where sda is OK, and sdb has been replaced) 2. reboot (or manually fix with mdadm --manage /dev/md[012] --add /dev/sdb[123]) Losses: -no hardware RAID controller: * drive array might not survive power loss or system crash as cleanly as with hardware controller. * Also, no alarm to scream its head off when a drive fails. * No hardware offload for consistent performance when CPUs are maxed out at 100% * no dedicated disk cache (shares filesystem buffer cache) -no fine-grained control over raid types & layout -single root filesystem -maximum of 4 primary partitions; more than 4 MD devices will require extended partitions -no LVM/EVMS, therefore more difficult to change partitions later Overall, this scheme works very well for me. It's not for everyone, but this matches my hardware and my workload almost perfectly - my servers generally are doing CPU work *or* I/O work but not both simultaneously, I have lots of RAM for buffers, I have dual CPUs which helps the performance stay consistent. -Adam Thompson Divisional IT Department, St. James-Assiniboia School Division 150 Moray St., Winnipeg, MB, R3J 3A2 athompson@xxxxxxxx / tel: (204) 837-5886 x222 / fax: (204) 885-3178 |
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