MD Series: MD3 arrays support SCSI UNMAP or TRIM commands
Summary:
This article covers information about "SCSI UNMAP or TRIM" commands on MD3 RBODs. It also contains additional information of article 000137759 (SLN156240) "Dell Solid State Drive
(SSD) FAQ with Servers (PowerEdge) and Storage."
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Background: Write amplification is defined as follows:
The write amplification factor is the amount of data the SSD controller has to write in relation to the amount of data that the host controller wants to write. A write amplification factor of 1 is perfect, it means you wanted to write 1 MB and the SSD's controller wrote 1 MB. A write amplification factor greater than 1 is not desirable, but is an unfortunate fact of life. The higher your write amplification, the quicker your drive will wear out and the lower its performance will be.
Write-amplification is a measure of the ratio of the number of flash writes relative to the host write. As an example, if 2 flash writes are generated per host write, the write amplification is also 2. A technique that is called overprovisioning improves the garbage collection efficiency, reducing write-amplification.
The following methods are used to correct write amplification on SSD drives.
TRIM is a SATA command that enables the operating system to tell an SSD which blocks of previously saved data are no longer needed as a result of file deletions or volume formatting. When an operating system replaces an LBA, as with an overwrite of a file, the SSD knows that the original LBA can be marked as stale or invalid and it will not save those blocks during garbage collection. If the user or operating system erases a file (not remove parts of it), the file will typically be marked for deletion, but the contents on the disk are never erased. Because of this, the SSD does not know that it can erase the LBAs previously occupied by the file, so the SSD will keep including such LBAs in the garbage collection.
TRIM command for SATA or UNMAP for SCSI -- The operating system must send these commands which tell the storage device which sectors contain invalid data. SSDs consuming these commands can then reclaim the pages containing these sectors as free space when the blocks containing these pages are erased instead of copying the invalid data to clean pages.
Cause
See Symptom section.
Resolution
Are SCSI UNMAP or TRIM commands supported on the MD arrays? UNMAP command is used only with reclaiming space when using Thin Provisioned Virtual Disks.
This started with 08.25.05.60 controller firmware for Tribes (MD34xx and MD38xx) and is not supported with Hogs (MD32xx and MD36xx) arrays.
MD Series, Dell PowerVault MD3200/MD3220-Windows HA Cluster, Dell PowerVault MD3200i and MD3220i with Windows HA Clusters, Dell PowerVault MD3600f/3620f Windows HA Cluster, Dell PowerVault MD3600i/3620i Windows HA Cluster
, Dell PowerVault OEM Ready MD34XX and MD38XX, PowerVault MD3200
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