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 Post subject: WD's OptiNAND and RRO and ATI metadata
PostPosted: October 31st, 2021, 15:08 
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Western Digital Unveils 20TB HDD with OptiNAND Technology:
https://www.tomshardware.com/news/western-digital-unveils-optinand-architecture-for-hdds

Quote:
Modern hard drives store gigabytes of metadata on spinning media since it cannot be cost-effectively stored in local DRAM and serial NAND. HDDs store repeatable runout (RRO) metadata (the share of the position error signal that is repeatable for every spindle revolution) as well as write operations metadata at the track level to account for increased adjacent track interference (ATI). With OptiNAND, RRO and write operations are stored on the iNAND drive, which frees up space on the rotating media, faster metadata availability, and reduces the number of read/write metadata-related operations, which further improve performance (e.g., random read/write performance). Additionally, the EFD stores write operations at the sector level, which optimizes storage requirements and reduce the number of ATI refreshes to increase performance.

Where is this RRO and ATI metadata currently stored? Is RRO storage the function of ROM module 4Fh in some models ???

Quote:
In addition, the iNAND EFD can be used to store over 100MB of write cache data in case of emergency power off (EPO) event, which improves reliability of an OptiNAND-enhanced HDD. Normally, drives from Western Digital only store about 2MB of write cache data to serial flash. Furthermore, with an iNAND EFD onboard and appropriate firmware optimizations, HDDs with OptiNAND can reduce their latency.

Which models use the ROM(s) for write caching?

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: October 31st, 2021, 15:49 
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This is bullshit.

RRO data takes 8-16 bytes per surface, as it takes only a few polynomial coefficients to store, not the position offsets for all tracks and all wedges. Another point where things can (and will) go really wrong, coz loosing these results in complete data loss. I haven't read the article, just the post above btw.

pepe

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: October 31st, 2021, 16:28 
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I understand that RRO requires only a few coefficients (you can see the RRO1x Sin and Cos stuff in Samsung's terminal output), but what about ATI? I confess that I was unaware of this phenomenon until I read the Tom's Hardware blog.

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: October 31st, 2021, 18:02 
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https://blog.westerndigital.com/optinand/

Quote:
One of those performance enhancements is not exactly how fast something is done, but how often. Specifically, with adjacent track interference (ATI) refreshing, using NAND will cut down the frequency of refreshes needed.

In addition to adding disks to a drive to gain capacity, another proven way to add capacity is to increase density by squeezing write tracks closer together. Imagine narrowing the lanes of a highway to fit more lanes of traffic.

However, a hard drive can only write so many times before there is a risk of magnetic interference on adjacent sectors to the one being written. The number of times a sector receives interference, when a sector has been rewritten, and the position of the write head are key drive metadata in that are stored in DRAM. The precision of the metadata is limited in DRAM, so the drive must sometimes infer — or make an educated guess — as to what precise track and position the data was written. To make sure the correct data is being accessed, the drive will go back and re-read the data, and then rewrite tracks to prevent any data corruption.

This entire process, which actually occurs very quickly, is called an ATI refresh. As tracks are moved closer and closer together to increase capacity, the number of writes allowed before a refresh must decrease to prevent any data corruption. Doing more refreshes increases latency, since the drive must take time to read and rewrite more often.

“It used to be, not that many generations ago, that you could write 10,000 times before needing to refresh sectors on either side,” Hall said. “And then as we pushed the tracks closer and closer together, it went to 100 then 50 then 10, and now for some sectors, it’s as low as six.”

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 12:05 
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i understand the need of refreshing adjacent tracks, but have no idea how using NAND would reduce the need for it. Unless it writes frequently written stuff in the nand, but hey, that's a NVC, which i can't believe they use a serial NAND for that (which the article mentions).
Quote:
Normally, drives from Western Digital only store about 2MB of write cache data to serial flash.

??????
never seen that. and to serial flash???

So i am seriously suspicious about the credibility of that article.

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 12:11 
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pepe wrote:
i understand the need of refreshing adjacent tracks, but have no idea how using NAND would reduce the need for it. Unless it writes frequently written stuff in the nand, but hey, that's a NVC, which i can't believe they use a serial NAND for that (which the article mentions).
Quote:
Normally, drives from Western Digital only store about 2MB of write cache data to serial flash.

??????
never seen that. and to serial flash???

6TB WD6001FSYZ-01SS7B1 PCB with 7 ROMs:
https://hddoracle.com/viewtopic.php?f=3&t=2762

    WD6001FSYZ-01SS7B1
    EIGERRE <-- family
    7200
    128M
    SATA3
    6GB/S
    6.0 TB
    10HD
    RE (RAID EDITION)

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 13:03 
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checked the dumps and it does not look like being a cache to me

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 13:15 
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pepe wrote:
checked the dumps and it does not look like being a cache to me

Likewise, but what is it?

BTW, I would think that SMR models would the most likely candidates for an NV write cache.

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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 14:25 
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dunno either. But caching to SPI flash sounds a bit steep to me.
ATI refresh in SMR drives is pretty much non-existent anyway.


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 Post subject: Re: WD's OptiNAND and RRO and ATI metadata
PostPosted: November 1st, 2021, 14:38 
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Toshiba's documentation talks about power loss data protection for their SMR drives, and they have 4 ROMs, IIRC. Toshiba calls it "Persistent Write Cache".

https://forum.hddguru.com/viewtopic.php?f=1&t=37261&p=275681&hilit=toshiba+power+loss+protection#p275681

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