Sensible size for hard drive

What is considered a sensible size of hard drive (Filecore format) these days on modern RISC OS platforms like Raspberry Pi 4, etc.? I have a lot of drives at 240GB or thereabouts and I cannot remember whether this was an actual limit once, or a sweet spot in the trade-off between capacity, LFAU, etc.

256GB is the Filecore limit with 512 byte sectors, 2TB with 4K sectors. So 240GB is fine.

As 240GB seems to be the smallest size of SSD readily available these days, I was wondering whether it makes any sense to go higher, as we do need a lot of space for RiscOSM map data conversions.

Regarding the Filecore limit, does that mean if you go higher than 256GB you have to go to 4K sectors?
Edit after reading other thread: yes, and as you have no choice as to sector size, you have to be careful what kind of drive you buy.

What impact does that have on the storage of smaller files? Wouldn’t the LFAU be higher than 512 bytes anyway, even for a 240GB disc?
Edit: yes, the LFAU has the bigger impact, and will be higher than 4K anyway for discs of this size.

Ah, I’ve found a thread from six years ago which explains things quite well, but I’ve no idea from that which machines support the FileCore changes that are mentioned about growing the idlen to 21 bits.

I guess it’s probably just how new the OS is, as Sprow said: “FileCore 3.75 and HForm 2.74 are the versions to check for (and DiscKnight 1.53)”.

An article in Archive 27:1 talked inter alia about idlen. You can switch between 512B and 4k sectors on a SSD drive under NVMe using either a RISC OS or a Linux utility. Most drives will allow you to do this. SATA does not let you do this.

FileCore (from RISC OS 3.80) used a 19-bit value (idlen=19) to identify a disc object, so there can be no more than 2¹⁹ objects, each requiring 20 bits in the map, to allow for the extra terminating bit. The granularity of a partition is the minimum separation between each disc object, given by the formula (idlen + 1)*LFAU. There can be no more than 2^idlen objects in the partition. This sets the minimum value of LFAU which is one parameter chosen when formatting a partition.

RISC OS 5.22 extended the maximum sector size to 4k (maximum partition size now 2TB). RISC OS 5.24 extended idlen to 21 bits so the granularity of a 2TB partition became 1408kB and on a 256GB 4k disc became 176k.

A 1TB partition on an NVMe drive will have an LFAU of at least 16k. Hope this helps. The speed of an NVME disc using 4k sectors is better (just) than a SATA disc. However a ‘Loader’ partition on a 4k disc cannot be shared between RISC OS and Linux as DOSFS is 512 sectors only.

A speed comparison can be read here

How can I find out what the LFAU value is for an existing drive? It’s easy enough to set it when formatting using HForm but I don’t want to format the drive, I just want to find out what the LFAU is.

I wondered whether Jon Abbott’s partition manager might show it as part of the information, but it doesn’t appear to.

Run DiscKnight with the Verbose option. It’ll say up the top something like:

  Minimum object size      : 21.0K

How can I find out what the LFAU value is for an existing drive?

Run DiscKnight to check the disc. The “Boot block – Boot Record” section will contain an entry “Bytes per map bit (LFAU)”.

NVMe4kFmt will tell you both the sector size (LBA) and LFAU of an existing NVMe drive.