SSD P4 (PATA) Solid State Drive
Product Manual
Rev 0.4
© 2010 SanDisk® Corporation
11
Document No. 80-11-XXXX1
1.7
Non-Volatile Write Cache
The third generation of modular SSD, the SSD P4 supports a unique feature to
improve random write performance and ensure very positive user experience.
Studies show that modern operating systems mostly access the storage device using
small access blocks, with the majority being a 4KB access blocks.
The small logical access blocks conflict with the physical block structure (>1MB) for
the newer generation flash memory technology. Therefore, to bridge this difference,
SSD P4 employs a non-volatile SLC Flash write cache, nCache
TM
The cache size of the SSD P4 is over 500MB
. The nCache™ is
used to accumulate these small writes (called segments) at high speed and then
flush & consolidate them to larger MLC section of the NAND Flash memory array in
the background during idle time.
4
Also nCache™ is a non-volatile cache and hence there is no exposure to data loss
- an order of magnitude larger than
other competing solutions that use DRAM based cache. Due to its large size, for most
part of daily user activity nCache™ never overflows and the end users experience
high SLC burst performance rather than the sustained performance. Once the
nCache™ becomes full, the performance of SSD P4 drops to the steady state
condition.
5
1.8
Power Management
.
Although power consumption is not actively managed by a PATA host, the SSD P4
supports an advanced power management system. This system is internally
managed (i.e., requiring no special interaction with the host) in order to agressively
manage and preserve power consumption.
Upon completion of a command, the P4 will start an internal timer and, after 20ms
without receiving another command, will automatically enter an intermediate low-
power mode (i.e., Idle time power mode). In order to further enhance the P4’s
performance, the flash management can utilize this idle time for performing internal
house-keeping operations. Doing those activities, i.e. – freeing up space in the
nCache, while the card is idle will not increase overall power consumption, since
these operations will have to be executed eventually. Executing them during idle
time will prevent stalling the host while they are taking place and will enable a better
utilization of the Data Cache, as it will have additional free space when the host will
access the device. At the moment a new command is received from the host the
internal operations will be terminated and the host command will be serviced with
minimal delay. If no command is received after 30 seconds, execution of the house-
keeping activities will terminate and the lowest-power mode will be entered.
4
The cache size scales with capacity. For 32GB product the cache size is over 500MB.
5
In order to prevent any data loss during removal of the power supply, the time it takes the input voltage
to drop from the nominal voltage down to 2V must be at least 5ms.
