Information Technology Reference
In-Depth Information
To improve durability in the face of wear from writes and disturbs from
reads, flash devices make use of a number of techniques:
Error correcting codes. Each page has some extra bytes that are used
for error correcting codes to protect against bit errors in the page.
Bad page and bad erasure block management. If a page or erasure
block has a manufacturing defect or wears out, firmware on the device
marks it as bad and stops storing data on it.
Wear leveling. As noted above, rather than overwrite a page in place,
the flash translation layer remaps the logical page to a new physical page
that has already been erased. This remapping ensures that a hot page
that is overwritten repeatedly does not prematurely wear out a particular
physical page on the flash device.
Wear leveling moves a ash device's logical pages to dierent physical
Denition: wear leveling
pages to ensure that no physical page gets an inordinant number of writes
and wears out prematurely. Some wear leveling algorithms also migrate
unmodified pages to protect against read disturb errors.
Spare pages and erasure blocks. Flash devices can be manufactured
with spare pages and spare erasure blocks in the device. This spare ca-
pacity serves two purposes.
First, it provides extra space for wear leveling: even if the device is logically
\full" the wear leveling rmware can copy live pages out of some existing
erasure blocks into a spare erasure block, allowing it to flash those existing
erasure blocks.
Second, it allows bad page and bad erasure block management to function
without causing the logical size of the device to shrink.
In addition to aecting reliability, wear out aects a ash device's perfor-
mance over time.
First, as a device wears out, accesses may require additional retries, slowing
them. Second, as spare pages and erasure blocks are consumed by bad ones, the
wear leveling algorithms have less spare space and have to garbage collect live
pages|copying them out of their existing erasure blocks|more frequently.
Example: Intel 710 Series Solid-State Drive Figure 12.6 shows some
key parameters for an Intel 710 Series solid state drive manufactured in 2011.
This drive uses multi-level NAND flash to get high storage densities. Normally,
multi-level flash is less durable than single-level, but this Intel drive uses sophis-
ticated wear leveling algorithms and a large amount of spare space to provide
high durability.
