Date: Thu, 21 May 2015 08:38:13 -0700 From: Chris Stankevitz <chrisstankevitz@gmail.com> To: freebsd-questions <freebsd-questions@freebsd.org> Subject: SLOG and SSDs: are "super" capacitors really needed? Message-ID: <CAPi0psuykA-uKWQ7XTszRk3oNzPXPd%2BXXVgxwJN1tFEAwD0buQ@mail.gmail.com>
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When sync data is written to the ZIL (or in my case to a SLOG), ZFS waits for the write to be "completed" before continuing. Once the write has "completed", the sync data is considered written, even if it has not yet made it to the real storage devices. Written data has "completed" when the ZIL device (SLOG) reports that the data has been written. Question: do SSD drives report the write has "completed" only after the data has been burned into non-volatile storage? If so, then why do people say a good SLOG SSD has "super capacitors" that allow the drive to continue functioning for a short time after a power failure? It seems to me that there are two scenarios, none of which need super capacitors: 1. A transaction is completely written to the SLOG, but not the storage devices, and the power goes out. No problem, data will write to storage when the pool is imported. 2. A transaction is partially written to the SLOG, but not the storage devices, and the power goes out. No problem, the transaction will be lost and the pool will be imported with the previously committed data/transaction. I don't see a scenario where a power-outage causes a "corrupted transaction" to be posted. Now if an SSD reports data "written" before it makes it to non-volatile storage, then that is another story... but I cannot imagine a HDD manufacturer advertising data written that is not actually written (or guaranteed to be written even in the face of a power outage). Thank you, Chris
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