[kwlug-disc] Flush to persist on Apple's NVMe drives

[Doug Moen]

F_FULLSYNC on Mac hardware may be slow, but at least it flushes the cache and stores the data in nonvolatile storage.

Not so much with some non-Apple NVMe SSDs. But I bet they benchmark well.

https://twitter.com/xenadu02/status/1495693475584557056

> The models that never lost data:
> Samsung 970 Evo Plus: MZ-V7S2T0, 2021.10
> WD Red: WDS100T1R0C-68BDK0, 04Sept2021

On Sat, Feb 19, 2022, at 11:37 AM, Doug Moen wrote:
> One clarification missing from that thread is that MacOS fsync() has 
> worked this way since 2004, and all of the databases with MacOS ports 
> already know to use F_FULLSYNC instead of fsync().
>
> So the new information is just how incredibly slow F_FULLSYNC is on the 
> M1. It's as if some intern implemented F_FULLSYNC using simple brute 
> force, and no engineering resources were allocated to testing how fast 
> it is, since apparently Apple doesn't use this operation in their own 
> code.
>
> A related issue, there's an unanswered question about whether Apple's 
> file system maintains file system integrity if there is an unexpected 
> power loss. This isn't an issue on laptops due to the battery. But it 
> may affect the mac mini, imac and mac pro. Somebody claimed that the 
> API for file system write barriers (ensuring that blocks are written 
> out in the correct order) is just as slow as F_FULLSYNC. So it's not 
> clear to me if Apple is using write barriers for file system integrity.
>
> On Fri, Feb 18, 2022, at 11:17 PM, Mikalai Birukou via kwlug-disc wrote:
>> Performance versus need to persist data in observations of Apple's NVMe 
>> drives: https://twitter.com/marcan42/status/1494213855387734019
>>
>>
>>
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