wiki:ssd

Version 1 (modified by Jamie McClelland, 5 years ago) (diff)

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Solid State Drives

To help alleviate slowness from a lot of disk reads and writes, we are in the processing of adding solid state drives to our existing servers. Solid state drives (SSDs) are significantly faster than the traditional spinning SATA drives we currently have installed.

A major source of the disk read/write contention comes from access to MySQL databases. This contention slows everything down and it is one of the significantly bottle necks when a web site that is driven by a MySQL database feels slow to load.

Therefore, our strategy is to move most MySQL database partitions to the newly installed solid state drives. That should allow MySQL powered web sites to load much faster, while also reducing the disk contention on the other partitions.

Technical Overview

Like all disks, solid state drives are installed in a RAID configuration for redundancy and are also encrypted.

They are added to their own dedicated volume group on the host server (typically named: vg_HOST1 - as opposed to vg_HOST0 for the volume group with SATA disks).

Just as each guest is allocated a logical volume named after the guest, each guest is additionally allowed a logical volume from the SSD volume group (also named after the guest). So, a guest may have both:

  • vg_wiwa0-jacobs
  • vg_wiwa1-jacobs

In the guest, the initial disk shows up as /dev/sda and the second disk shows up as /dev/sdb.

For the normal SATA disks - they are added to a logical volume group on the guest and then further allocated.

However, with the SSD disks - a single partition is created from the block device and a filesystem is installed directly on it.

Due to limitations of SSD, the operating system must communicate to the disk which sectors are free for writing (trim/discards), we have to configure all the disk layers specifically to pass these "disard" messages.