Say I have some partitions; one on a SATA drive and one on an SSD. I want to create a "virtual" hybrid disk. Is this possible?
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5 Answers
If you have a set of files for which you need low latency, you can establish a RAID-1 volume that mirrors the SSD content on a hard disk. Declare the hard disk component as “write-mostly”, so that the SSD will be favored when reading. For example, if sda is your SSD and sdb is your hard disk, create a partition sda1 that covers the whole drive, create a partition sdb1 of the same size, and create a RAID volume with
mdadm --create -l 1 -n 2 /dev/md0 /dev/sda1 --write-mostly /dev/sdb1 answered Jan 24, 2013 at 23:11
Gilles 'SO- stop being evil'
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- This is seriously a good catch. It's a straight answer to this on AU: Can I cache an mdadm raid with an ssd? (in which I now mentioned your answer in mine)gertvdijk– gertvdijk2013-02-06 21:17:42 +00:00Commented Feb 6, 2013 at 21:17
- Note this means the array obviously needs to include the SSD, so using a 120GB SSD with a 2TB HDD is going to get you a 120GB array -- not what people are usually after when they want SSD caching.cdhowie– cdhowie2019-09-05 22:41:57 +00:00Commented Sep 5, 2019 at 22:41
There is flashcache. It is used by Facebook to accelerate DB-storage systems.
I haven't tried it yet, so your mileage may vary.
Since Linux 3.9 dm-cache has been merged into the mainline tree. As per the documentation, it's specifically designed for this purpose.
It aims to improve performance of a block device (eg, a spindle) by dynamically migrating some of its data to a faster, smaller device (eg, an SSD).
It will also provide write caching to some extend:
If writeback, the default, is selected then a write to a block that is cached will go only to the cache and the block will be marked dirty in the metadata.
About the virtual hybrid disk you'd like to see:
This device-mapper solution allows us to insert this caching at different levels of the dm stack, for instance above the data device for a thin-provisioning pool.
In other words, the dm-stack lets you use any block device/partition to become a single cached virtual disk as a storage device, just like a /dev/md0 in a software raid set.
LVM also allows this:
Hybrid volumes can be created using the dm-cache target, which allows one or more fast storage devices, such as flash-based SSDs, to act as a cache for one or more slower hard disk drives.