Unlimited-Data. moved to lab.itbee.vn : Long tailed workloads and the return of hierarchical storage management

Long tailed workloads and the return of hierarchical storage management: "

Hierarchical
storage management (HSM) also called
tiered storage management is back but in a different form. HSM exploits the access
pattern skew across data sets by placing cold, seldom accessed data on slow cheap
media and frequently accessed data on fast near media. In
old days, HSM typically referred to system mixing robotically managed tape libraries
with hard disk drive staging areas. HSM was actually never gone – its just a very
old technique to exploit data access pattern skew to reduce storage costs. Here’s
an old unit from FermiLab.

Hot data or data currently being
accessed is stored on disk and old data that has not been recently accessed is stored
on tape. It’s a great way to drive costs down below disk but avoid the people costs
of tape library management and to (slightly) reduce the latency of accessing data
on tape.

The basic concept shows up anywhere where
there is data access locality or skew in the access patterns where some data is rarely
accessed and some data is frequently accessed. Since evenly distributed, non-skewed
access pattern only show up in benchmarks, this concept works on a wide variety of
workloads. Processors have cache hierarchies where the top of the hierarchy is very
expensive register files and there are multiple layers of increasingly large caches
between the register file and memory. Database management systems have large in memory
buffer pools insulating access to slow disk. Many very high scale services like Facebook
have mammoth in-memory caches insulating access to slow database systems. In the Facebook
example, they have 2TB of Memcached in front of their vast MySQL fleet: Velocity
2010.

Flash memory again opens up the opportunity
to apply HSM concepts to storage. Rather
than using slow tape and fast disk, we use (relatively) slow disk and fast NAND flash.
There are many approaches to implementing HSM over flash memory and hard disk drives.
Facebook implemented Flashcache which
tracks access patterns at the logical volume layer (below the filesystem) in Linux
with hot pages written to flash and cold pages to disk. LSI
is a good example implementation done at the disk controller level with their CacheCade product.
Others have done it in application specific logic where hot indexing structures are
put on flash and cold data pages are written to disk. Yet another approach that showed
up around 3 years ago is a Hybrid
Disk Drive.

Hybrid drives combine large, persistent
flash memory caches with disk drives in a single package. When they were first announced,
I was excited by them but I got over the excitement once I started benchmarking. It
was what looked to be a good idea but the performance really was unimpressive.

Hybrid rives still looks like a good idea
but now we actually have respectable performance with the Seagate
Momentus XT. This part is actually
aimed at the laptop market but I’m always watching client progress to understand what
can be applied to servers. This finally looks like its heading in the right direction. See
the AnandTech article on this drive for more performance data: Seagate's
Momentus XT Reviewed, Finally a Good Hybrid HDD.
I still slightly prefer the Facebook FlashCache approach but these hybrid drives are
worth watching.