Let's see if Crucial will finally have CAPs on these... unlike the MX100 blunder.
That is mostly a misunderstanding.
Power-safe capacitors are there to protect the mapping tables from becoming inconsistent, as well as preventing damage to the NAND when a write/erase-cycle is interrupted.
The Intel 320 has capacitors that protect the full write-back. No DRAM since this SSD does not have DRAM chip. The 320 uses Intel's own controller and it has 192KiB of SRAM buffercache. SRAM uses less power than DRAM and thus providing power-safe capacitors to protect the SRAM is much cheaper than protecting a DRAM chip.
So most SSDs actually do not protect the write-back buffercache but instead concentrate on protecting the mapping tables (FTL) from becoming inconsistent with the stored data.
Samsung does this using software; by journaling the mapping tables and rolling them back whenever power has been interrupted (POR - Power-On Recovery).
Crucial M500/M550/M600/MX100/MX200 does this using a hardware feature - the power-safe capacitors. This has the advantage of not having to rewind the whole SSD to an earlier state like Samsung does, which can have extreme consequences.
While the write-back in DRAM is not protected, that is actually not a big deal. Virtually no storage device protects its DRAM and since long time filesystems have been designed to cope with this since Windows NT4 (NTFS). You have to go back to the Windows 95 era with 'Scandisk' to find problems with this. As FAT did not have any way to protect against dirty write-back, filesystem damage was very common. But not so anymore for any 2nd generation filesystem like NTFS, UFS+SU or Ext3/4.
The important metadata is written as synchronous write, meaning that no other I/O will be sent until the sync write has been completed. This works by writing to the device and sending a FLUSH CACHE command afterwards. The effect is that only one I/O will be queued in DRAM and it provides atomicity for the filesystem: either that sync write made it through, or not. If not, no other data will be written as well, which is what protects the filesystem against becoming inconsistent.
Conclusion is that Crucial's power-safe capacitors work as intended and is one of the key reasons to select Crucial over other SSDs. I should note that Intel S3500/S3700 SSDs do the exact same thing. It's way better than the software protection that Samsung uses; which makes the SSD unsuitable for anything but consumer usage.
Facebook
Twitter