getting started faq: a new "how to organise files" answer

src/Getting-started-FAQ.md

@@ -26,21 +26,109 @@ Circa 2022 I hope it's fair to say, without my bias showing:
 
 SM on new user experience (2023): https://fosstodon.org/@simonmic/110120922501551538
 
-### How to organise files ? Should I split the journal into many files ?
+### How to organise the journal file(s) ?
 
-All in one file (or one file per year) and ordered by date is simplest
-and creates the fewest headaches (balance assertions/assignments,
-scope of directives, where to put entries, finding things..).
+Most PTA apps support reading from multiple files at once,
+by specifying them all on the command line or by having
+files `include` other files. Should you use this feature ?
 
-If you use emacs: it’s possible to insert org headings (which are
-comments to h/ledger) and then use org-mode or org-minor-mode to
-collapse/expand/navigate sections of your journal.
+Just one file, and with transactions strictly ordered by date, is the simplest setup.
+Especially while you are learning about PTA bookkeeping and your app's features.
+It minimises choices and avoids having to learn how multiple files
+interact with directives, balance assertions, balance assignments, etc.
 
-Also in emacs: in ledger-mode, C-c C-f can give you a filtered view of
-just one account’s transactions. (But be careful, if you edit in the
-wrong place it will lose data.)
+As data grows, at some point you may want to split the journal into multiple files, eg because:
+
+1. Reports have become noticeably/bothersomely slow.
+   This might happen after months or years depending on your machine and your PTA app.
+
+2. Editing, finding, organising entries in the big file has become bothersome. 
+   This depends partly on your text editor's navigation features
+   (eg Emacs users can fold sections of the journal, fold all entries to one line,
+   filter the entries by a regexp and so on).
+
+3. Reports are showing too much history, and you would like to see just recent data by default.
+
+4. You would like to do some cleanup of old entries, without fear of affecting current reports.
+
+Perhaps the easiest and first thing to split out is `P` price records.
+You could put them all in a single prices file, but one file per currency/commodity is quite convenient.
+Eg `eur.journal`, `jpy.journal`, `aapl.journal` etc.
+
+You might want to organise other directives, such as account or commodity declarations or account aliases, in their own files.
+You'll need to check your PTA app's rules about how these directives affect other files, and how their sequence matters.
+Eg in `hledger`, account and commodity declarations can be kept in a sub-file, but account aliases must be in a top-level or parent file.
+
+Splitting files by account may be worth considering. 
+It makes things more awkward when you need to transfer between accounts in different files.
+But it can make sense where you have accounts with a lot of activity which don't interact much with the rest.
+
+In the above cases, all of the files are still read together as a single file set,
+and there is no reduction in data size or run time. For that, you must split by time.
+This adds some complications, and isn't necessary for everyone, so we don't recommend it from the start.
+
+But, most PTA users sooner or later will split files by time, and most of them will split by year.
+This allows you process just one year or a subset of years when running reports, which is more efficient.
+The new complication is how to arrange for correct account balances no matter which year file(s) you are processing.
+You can read more about that in the links below.
+
+So, putting these things together, here is a common and relatively simple multi-file layout, 
+with all files in the same directory for easy access:
+
+```
+2022.journal
+2023.journal
+2024.journal
+aapl.journal
+eur.journal
+jpy.journal
+...
+```
+
+And here is one with more files, 
+including per-year declarations (allowing for an evolving chart of accounts, eg),
+organised in per-year subdirectories:
+
+```
+2023/
+ 2022.journal
+ 2022-accounts.journal
+ 2022-commodities.journal
+ 2022-payees.journal
+ 2022-tags.journal
+2023/
+ 2023.journal
+ 2023-accounts.journal
+ 2023-commodities.journal
+ 2023-payees.journal
+ 2023-tags.journal
+2024/
+ 2024.journal
+ 2024-accounts.journal
+ 2024-commodities.journal
+ 2024-payees.journal
+ 2024-tags.journal
+aapl.journal
+eur.journal
+jpy.journal
+...
+```
+
+See also:
+
+- https://ledger-cli.org/doc/ledger3.html#Archiving-Previous-Years
+- https://hledger.org/dev/hledger.html#close
+- https://www.reddit.com/r/plaintextaccounting/comments/d69slp/how_do_you_manage_multiple_accounts_in/ (2019)
+- https://www.reddit.com/r/plaintextaccounting/comments/ay8ec0/limits_on_ledger_cli_file_size/ (2019)
+- https://www.reddit.com/r/plaintextaccounting/comments/dbjizl/how_big_is_your_ledgerjournal_file/ (2019)
+- https://www.reddit.com/r/plaintextaccounting/comments/fcx8ls/multifile_ledger_setup/ (2020)
+- https://www.reddit.com/r/plaintextaccounting/comments/lhdy5c/how_to_handle_yearly_journals/ (2021)
+- https://www.reddit.com/r/plaintextaccounting/comments/ukfxqo/double_entry_balances_between_years_in_ledgercli/ (2022)
+- https://www.reddit.com/r/plaintextaccounting/comments/184fz8r/is_it_possible_to_get_balance_from_multiple/ (2023)
+- https://www.reddit.com/r/plaintextaccounting/comments/18zxlbn/hledger_year_closing/ (2024)
+- https://groups.google.com/g/hledger/c/G0LLoV4PnBM/m/A2_5fXZQAgAJ (2023
+- https://github.com/simonmichael/hledger/issues/2151
 
-https://www.reddit.com/r/plaintextaccounting/comments/d69slp/how_do_you_manage_multiple_accounts_in/
 
 ### How big is your ledger/journal file ?