2006-10-13 20:10:56

�� /HCL / Xranenie Dannyx / Software RAID �� 2006-10-13 20:10:56 � 2007-04-21 12:43:35

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�� RAID1

PS: �� M24 � 4.0beta20070413, �� Software RAID1; �� , �� degraded raid, � �� — �� : �� , �� , � �� , �� raid superblock.

RAID5

�� RAID4/5/6 �� (hda/sda «�� » — �� , �� -�� -�� , �� , �� — �� -�� — � �� ) �� (� �� PATA/SATA-�� 63, �� softraid).
�� Aligning ASM Disks on Linux � �� (�� , �� raid5 stripe alignment linux �� ...):
If you switch to sector mode, you will see what the true start offset is:
Command (m for help): u
Changing display/entry units to sectors
Command (m for help): p
Disk /dev/sdc: 513.5 GB, 513556348928 bytes
255 heads, 63 sectors/track, 62436 cylinders, total 1003039744 sectors
Units = sectors of 1 * 512 = 512 bytes

Device Boot Start End Blocks Id System

/dev/sdc1 63 1003034339 501517138+ 83 Linux
Notice the start sector, 63th sector. That�s the 31.5 kB boundary. This value does not align with any stripe size. Stripe sizes are usually multiples of 2 and above 64 kB.
The result is, every so often, a block will be split between 2 separate hard disks, and the data will be returned at the speed of the slower (busier) device.
Assuming the typical 64 kB stripe (way too low, as I will discuss in another blog), and 8 kB database block size, every 8th block will be split between 2 devices. If you do the math, that�s about 12% of all your I / O?. Not a significant number, but when you consider how discs are arranged in RAID 5, instead of a logical write being 2 reads and 2 writes (data+checksum, update, then write them back), each logical write could be 4 reads and 4 writes, significantly increasing your disk activity.
The solution?
Before you create your partitions, switch to �sector� mode, and create your partitions at an offset that is a power of 2.
I typically create my partitions at the 16th megabyte (32768th sector). Essentially, I �waste� 16 MBs, but gain aligned I / O? for stripe width of up to 16 MB.
The procedure to create an aligned disk is:
[]# fdisk /dev/sdg
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
When building a new DOS disklabel, the changes will remain in memory only, until you decide to write them. After that, of course, the previous content won't be recoverable.
Command (m for help): u
Changing display/entry units to sectors
Command (m for help): p
Disk /dev/sdg: 143.4 GB, 143457779712 bytes
255 heads, 63 sectors/track, 17441 cylinders, total 280190976 sectors
Units = sectors of 1 * 512 = 512 bytes
Device Boot Start End Blocks Id System
Command (m for help): n
Command action
e extended
p primary partition (1–4) p
Partition number (1–4): 1
First sector (63–280190975, default 63): 32768
Last sector or +size or +sizeM or +sizeK (32768–280190975, default 280190975):
Using default value 280190975
Command (m for help): p
Disk /dev/sdg: 143.4 GB, 143457779712 bytes
255 heads, 63 sectors/track, 17441 cylinders, total 280190976 sectors
Units = sectors of 1 * 512 = 512 bytes
Device Boot Start End Blocks Id System
/dev/sdg1 32768 280190975 140079104 83 Linux
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
This way, the disk is aligned. When it is aligned at least on 1MB, then the ASM files will be also aligned at the 1MB boundary.
This alignment also applies to ext3 file systems. This file system takes it a step further, allowing you to provide the array stride as a parameter during creation, optimizing writing performance (I have not tested this). Look in the man pages for more information.

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�� RAID1...

FreeSource - HCL/XranenieDannyx/SoftwareRAID

2007-04-21 12:58:16

2007-04-21 12:54:47

2007-04-21 12:43:35

2006-10-13 20:10:56

�� RAID1

RAID5

FreeSource - HCL/XranenieDannyx/SoftwareRAID

2007-04-21 12:58:16

2007-04-21 12:54:47

2007-04-21 12:43:35

2006-10-13 20:10:56

���������� RAID1

RAID5

�� RAID1