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This section will mention some of the hardware concerns involved when running software RAID. References to IDE and SCSI have been deleted, all recent drives are SATA.
If you are going after high performance, you should be using SSDs (or hybrid drives), and make sure you match the performance of the drives to the performance of the bus. Many motherboards come with 6 SATA connectors so setting up a RAID is easy and affordable.
See also the section on bottlenecks.
Desktop and Enterprise drives
Disk drives now tend to come in two varieties, desktop drives from which most of the features needed for a decent raid have been deleted, and enterprise drives, which have the features but are designed to run 24/7. So if you want to run raid on a desktop system it's rather difficult to find a drive that is suitable.
TLER and SCT/ERC
TLER (Time Limited Error Recovery) is a WD creation, which means that drives will return within 7 seconds. Having introduced it, WD subsequently disabled it on most desktop drives, although it is enabled by default on enterprise drives.
SCT/ERC is the generic specification implemented by TLER.
If it's available this feature needs to be enabled. If it isn't enabled or available, the linux defaults will interact badly with the drive, and a single drive failure will usually take down the array.
This command will tell you what the drive is capable of. If possible, it would be wise to see the output of it on the drive(s) you are thinking of buying. The following is the output from my laptop's Toshiba drive. Note especially where it says SCT Error Recovery Control is supported.
crappit:/home/anthony # smartctl -x /dev/sda smartctl 6.2 2013-11-07 r3856 [x86_64-linux-4.1.27-27-default] (SUSE RPM) Copyright (C) 2002-13, Bruce Allen, Christian Franke, www.smartmontools.org === START OF INFORMATION SECTION === Device Model: ST2000LM003 HN-M201RAD Serial Number: S321J9DG805231 LU WWN Device Id: 5 0004cf 2106b38eb Firmware Version: 2BC10001 User Capacity: 2,000,398,934,016 bytes [2.00 TB] Sector Sizes: 512 bytes logical, 4096 bytes physical Rotation Rate: 5400 rpm Device is: Not in smartctl database [for details use: -P showall] ATA Version is: ATA8-ACS T13/1699-D revision 6 SATA Version is: SATA 3.0, 6.0 Gb/s (current: 3.0 Gb/s) Local Time is: Tue Sep 20 00:05:59 2016 BST SMART support is: Available - device has SMART capability. SMART support is: Enabled AAM feature is: Disabled APM feature is: Disabled Rd look-ahead is: Enabled Write cache is: Enabled ATA Security is: Disabled, NOT FROZEN [SEC1] Wt Cache Reorder: Enabled === START OF READ SMART DATA SECTION === SMART overall-health self-assessment test result: PASSED General SMART Values: Offline data collection status: (0x00) Offline data collection activity was never started. Auto Offline Data Collection: Disabled. Self-test execution status: ( 0) The previous self-test routine completed without error or no self-test has ever been run. Total time to complete Offline data collection: (22740) seconds. Offline data collection capabilities: (0x5b) SMART execute Offline immediate. Auto Offline data collection on/off support. Suspend Offline collection upon new command. Offline surface scan supported. Self-test supported. No Conveyance Self-test supported. Selective Self-test supported. SMART capabilities: (0x0003) Saves SMART data before entering power-saving mode. Supports SMART auto save timer. Error logging capability: (0x01) Error logging supported. General Purpose Logging supported. Short self-test routine recommended polling time: ( 1) minutes. Extended self-test routine recommended polling time: ( 379) minutes. SCT capabilities: (0x003f) SCT Status supported. SCT Error Recovery Control supported. SCT Feature Control supported. SCT Data Table supported.
SATA Configuration (2011)
SATA is beginning to support a new feature called "port multipliers", which effectively multiplex several SATA disks onto the same host SATA port. this can decrease cabling concerns. it's also fairly common to see multi-port SATA controllers, which put 4 ports onto the connector originated by Infiniband; this makes it possible to create 24-port SATA controllers, for instance.
Hot Swap (2011)
Note: for description of Linux RAID hotplug support, see the Hotplug page.
Hot-swapping with SATA/SAS
SATA/SAS hotplug support is required by the SATA/SAS specifications, therefore SATA/SAS platform is the one where hotplug should be least problematic. But still, you can fall in non-compliance pitfalls, so read on before you start experimenting!
Hotplug support in mainboard/disk controller chipsets
Newer mainboard/disk controllers chipsets and their drivers usually support hotplug.
If the chipset is AHCI-compliant, it will be (probably) able to use the ahci kernel module providing hotplug and power managment support. The ahci module is present in the Linux kernel since 2.6.19.
But still, not all chipsets support hotplug. Also, some chipsets that could in theory support hotplug (but are not AHCI-compliant) don't have the necessary support in the linux kernel. For more information on SATA drivers' status, see http://ata.wiki.kernel.org/index.php/SATA_hardware_features.
Hotplug support in SATA/SAS disks
All current SATA and SAS drives that have the 15 pin SATA power connector are hotplug-ready. There might be some very old historical SATA disks with 4-pin Molex power connector which do not have the 15 pin SATA power connector. Such old drives should never be hotplugged directly (without a hotswap bay) otherwise you risk their damage.
Hotplug support by SATA/SAS cables
For protecting the disk circuitry during the hotplug, the 15-pin SATA/SAS power connector on the cable side must have 2 pins (pin nr. 4 and 12) longer than the others.
- on cable/backplane connector ("receptacle") side, pins 4 and 12 are longer and are called "staggered pins". These pins bring the GND to the disk before the other pins get attached, ensuring that no sensitive circuitry is connected before there is a reliable system ground
- on the device side, pins 3, 7, 13 are the staggered pins. These pins bring the 3.3V, 5V and 12V power to the precharge power electronics in the disk before the other power pins are atached.
Important warning Normal 15-pin SATA power cable receptacle, found in ordinary power supplies or computer cases, does not have pins 4 and 12 staggered! In fact, it is quite hard to find a hotplug-compatible SATA power receptacle. On the first sight, the difference is subtle, see pictures of several SATA receptacle types here before you try start playing hotplug games with your drive!
!!!! Please remember, that without the staggered GND pins on the SATA power cable receptacle, you risk the damage of your disk when doing hotplug/hot-unplug !!!!
The hotplug-compatible SATA power receptacle must be present in all SAS/SATA hotswap cages.
In case you don't have hotswap cage, but you do have 15pin hotplug-compatible SATA power receptacle, this should be the correct sequence for plugging and unplugging the disk:
- connect the 15pin power receptacle to the disk
- connect the 7pin data cable
- unplug the data cable from the disk
- unplug the power cable
Hot-swapping with SCA
With SCA, it is possible to hot-plug devices. Unfortunately, this is not as simple as it should be, but it is both possible and safe.
Replace the RAID device, disk device, and host/channel/id/lun numbers with the appropriate values in the example below:
- Dump the partition table from the drive, if it is still readable:
sfdisk -d /dev/sdb > partitions.sdb
- Mark faulty and remove the drive to replace from the array:
mdadm -f /dev/md0 /dev/sdb1 mdadm -r /dev/md0 /dev/sdb1
- Look up the Host, Channel, ID and Lun of the drive to replace, by looking in
- Remove the drive from the bus:
echo "scsi remove-single-device 0 0 2 0" > /proc/scsi/scsi
- Verify that the drive has been correctly removed, by looking in
- Unplug the drive from your SCA bay, and insert a new drive
- Add the new drive to the bus:
echo "scsi add-single-device 0 0 2 0" > /proc/scsi/scsi
(this should spin up the drive as well)
- Re-partition the drive using the previously dumped partition table:
sfdisk /dev/sdb < partitions.sdb
- Add the drive to your array:
mdadm -a /dev/md0 /dev/sdb1
The arguments to the "scsi remove-single-device" commands are: Host, Channel, Id and Lun. These numbers are found in the "/proc/scsi/scsi" file.
The above steps have been tried and tested on a system with IBM SCA disks and an Adaptec SCSI controller. If you encounter problems or find easier ways to do this, please discuss this on the linux-raid mailing list.
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